Peronia Fleming, 1822a

Dayrat, Benoit, Goulding, Tricia C., Apte, Deepak, Aslam, Sadar, Bourke, Adam, Comendador, Joseph, Khalil, Munawar, Ngo, Xuan Qu ảng, Tan, Siong Kiat & Tan, Shau Hwai, 2020, Systematic revision of the genus Peronia Fleming, 1822 (Gastropoda, Euthyneura, Pulmonata, Onchidiidae), ZooKeys 972, pp. 1-224 : 1

Genus Peronia Fleming, 1822a

Onchis Férussac, 1822: xxxi. Nomen oblitum.

Peronia Fleming, 1822a: 574; Fleming 1822b: 463. Nomen protectum.

Peronia Blainville, 1824: 280 [junior homonym of Peronia Fleming, not a reference of Peronia Fleming].

Eudrastus Gistel, 1848: x.

Paraperonia Labbé, 1934a: 196.

Scaphis Labbé, 1934a: 203.

Lessonia Labbé, 1934a: 213 [junior homonym of Lessonia Swainson, 1832, replaced by Lessonina Starobogatov, 1976].

Quoya Labbé, 1934a: 216.

Lessonina Starobogatov, 1976: 211.

Quoyella Starobogatov, 1976: 211 [unnecessary replacement name for Quoya Labbé, 1934a].

Type species.

Onchis : Onchidium peronii Cuvier, 1804, by monotypy.

Peronia Fleming: Onchidium peronii Cuvier, 1804, by monotypy.

Peronia Blainville: Peronia mauritiana Blainville, 1824, by original designation.

Eudrastus : Onchidium tonganum Quoy & Gaimard, 1832, by subsequent designation ( Baker 1938: 86).

Paraperonia : Paraperonia gondwanae Labbé, 1934a, by subsequent designation ( Starobogatov 1976: 211).

Scaphis : Onchidium astridae Labbé, 1934b, by subsequent designation ( Starobogatov 1976: 211).

Quoya : Quoya indica Labbé, 1934a, by monotypy.

Lessonina : Onchidium ferrugineum Lesson, 1831a, by monotypy.

Quoyella : Quoya indica Labbé, 1934a, by monotypy.

Etymology.

Onchis : After the Greek ὁ ὂγκος, oncos, which means mass, or tumor.

Peronia : After François Péron [1775-1810], zoologist of the Baudin expedition between 1800 and 1803, during which he collected the two slugs (from Mauritius and Timor) which Cuvier described as Onchidium peronii in 1804.

Eudrastus : Likely, although for unclear reasons, from the Greek εὖ, eu, for true, and δραστέoς, drasteos, a verbal adjective which means to be done.

Paraperonia : From the Greek παρα, para, meaning beside, and Peronia .

Scaphis : After the Greek ἡ σκᾰφίs, which means small boat ( Labbé, 1934a: 202).

Quoya : After the French naturalist Jean René Constant Quoy [1790-1869], a member of two circumnavigations from 1817 to 1820 with captain Freycinet and from 1826 to 1829 with captain Dumont d’Urville. Quoy and Joseph Paul Gaimard [1793-1858] described several species of onchidiids based on their collections in the southern seas. Quoyella has the same etymology.

Lessonina : After the French naturalist René Primevère Lesson [1794-1849], a member of a circumnavigation from 1822 to 1825 with captain Duperrey. Lesson described several species of onchidiids based on his collections in the southern seas, such as the type species of Lessonina , Onchidium ferrugineum , which he collected in West Papua, Indonesia. Labbé’s invalid name Lessonia was also dedicated to Lesson.

Gender.

Onchis : Masculine. Férussac did not specify the gender of Onchis which he did not combine with any specific name, and even the binomen Onchis peronii , which Férussac did not use per se, would not help in that respect. Because Onchis is derived from the masculine Greek noun ὁ ὂγκος, it is considered to be of masculine gender.

Peronia : Feminine. No gender was specified by Fleming, and the combination Peronia peronii does not help to determine it. Because no gender was originally specified or indicated and because Peronia ends in - a, it is treated as a name of feminine gender ( ICZN 1999: Article 30.2.4). Indeed, Peronia mauritiana , an early combination used by Blainville (1824: 281), shows that Peronia has always been treated as a name of feminine gender.

Eudrastus : Masculine. No gender was originally specified or indicated. Eudrastus ends in a word derived from a word of variable gender (a verbal adjective) and should be treated as masculine ( ICZN 1999: Article 30.1.4.2).

Paraperonia : Feminine. Gender of Peronia .

Scaphis : Feminine. The gender was not specified by Labbé, but his original combinations S. atra , S. carbonaria , S. lata , and S. punctata indicate that he treated Scaphis as a name of feminine gender, which is correct since Scaphis is derived from the feminine Greek noun ἡ σκᾰφίs.

Quoya : Feminine. The gender was not specified by Labbé, but his original combination Q. indica indicates that he treated Quoya as a name of feminine gender, which is assumed to be the gender of Quoyella as well.

Lessonina : Feminine. The gender was not specified by Starobogatov, and no gender was specified for Lessonia by Labbé. Labbé’s original combination Lessonia ferruginea indicates that he treated Lessonia as a name of feminine gender, which is assumed to be the gender of Lessonina as well.

Diagnosis.

Body not flattened. Dorsal gills present. Dorsal eyes present. No retractable, central papilla present. Eyes at tip of short ocular tentacles. Male opening below right ocular tentacle and to its left. Foot wide. Pneumostome median, on ventral hyponotum. Intestinal loops of types I or V. Rectal gland absent. Accessory penial gland present, with muscular sac. Penis with hooks.

Remarks.

Phylogenetic analyses show that all species of slugs with dorsal gills belong to the same clade (Figs 2 - 4 ). Seven generic names apply to that clade (excluding spelling mistakes, unjustified emendations, replaced names, and Peronia Blainville, 1824, a junior homonym of Peronia Fleming, 1822a). Note that the species name of a type species can be valid (such as Peronia peronii ), synonymous (such as Onchidium tonganum , junior synonym of P. peronii ), or even a nomen dubium (such as Quoya indica ). Remarks on the nomenclatural history of the genus Peronia follow a chronological order.

Cuvier (1804) described the first Peronia species as Onchidium peronii but did not mention the presence of dorsal gills. Nor did he illustrate them. He only described a mantle covered by small warts subdivided in even smaller warts. Dorsal gills are actually present on the dorsum of the type specimen of Onchidium peronii from Timor, but they are retracted, as most often seen in preserved specimens. Cuvier (1804: 41) also confessed that he would have believed O. peronii to be terrestrial, due to its pulmonary cavity "similar to that of reptiles", but that he regarded it as marine because Péron was certain to have collected it in seawater. But, Cuvier (1804: 41) adds: "I think at least that it comes to the surface to open its [pulmonary] hole, and naturally take air to breathe, as do our bulines [ Bulinus ] and our planorbes [ Planorbis ] which, although aquatic, breathe only air." Actually, Peronia slugs hide in crevices at high tide and only come out at low tide.

Cuvier (1804: 38) decided to classify his new species in Buchannan’s (1800) genus Onchidium because of the "extreme external resemblance" between O. peronii and Buchannan’s O. typhae , despite the fact that, according to Buchannan, sexes are separate in O. typhae , while Cuvier’s O. peronii is hermaphroditic. In his description of O. typhae , Buchannan (1800) wrote that slugs live in Bengal on leaves of Typha reeds and are "very nearly allied" to Limax , suggesting that they are terrestrial, although he did not mention the presence of a pulmonary cavity and did not clearly state whether the slugs were terrestrial or not. At any rate, authors considered that Buchannan’s (1800) O. typhae was not a marine species and Blainville (1817: 440) argued that Buchannan’s O. typhae was “generically” different from Cuvier’s O. peronii , and that Onchidium should be restricted to O. typhae . However, Blainville (1817) did not propose any new generic name for Cuvier’s (1804) Onchidium peronii .

In his Histoire naturelle générale et particulière des mollusques terrestres et fluviatiles, Férussac (1819: 80-82) agreed with Blainville (1817) that Cuvier’s (1804) Onchidium peronii was distinct from Buchannan’s (1800) O. typhae , but, like Blainville, he refrained from creating a new generic name for O. peronii . A year later, Fleming (1820: 616) stated that he thought Cuvier’s O. peronii should probably be classified in a different genus from Buchannan’s Onchidium : "This species [ Onchidium typhae Buchannan], however, if the description be accurate, differs essentially from the one described by Cuvier [ Onchidium peronii ], and would lead us to infer that a new genus would be necessary for the reception of the species of the last-mentioned naturalist."

The two generic names Onchis and Peronia were independently created in 1822 for O. peronii , respectively by Férussac (1822) and Fleming (1822a, b) who both follow Blainville’s (1817) argument according to which a marine and hermaphroditic species ( O. peronii ) cannot be classified in the same genus as a terrestrial species with separate sexes ( O. typhae ). Interestingly, neither Férussac (1822) nor Fleming (1822a, b) mention dorsal gills (which, again, Cuvier did not mention in the original description of O. peronii ). Dorsal gills were first illustrated by Savigny (1817: pl. 2, fig. 3.5) in the Description de l’Egypte for slugs from the Red Sea; for a collation, see Baring (1838) and Sherborn (1897). However, gills remained completely unnoticed because the explanation of Savigny’s plate was published nearly ten years later by Audouin (1826: 18-20).

The exact date of publication of Onchis is 13 April 1822 (when the pages xxv-xlvii were published); a collation for Férussac’s (1821-1822) Tableaux can be found in Coan and Kabat (2019). Férussac (1822: xxxi) clearly distinguishes two genera of onchidiids: "Genre I. Onchide, Onchis ; Onchidium , Cuvier, Ocken. (Marin.)" and "Genre II. Onchidie, Onchidium , Buchannan, Ocken." Note that Oken (1815: 307), to which Férussac (1822) refers, merely listed Onchidium typhae and O. peronii . According to Férussac, Onchis clearly refers to Cuvier’s Onchidium peronii , supposedly marine and living underwater, and Onchidium is restricted to Buchannan’s Onchidium typhae , thought not to live underwater. In the Tableau systématique de la famille de limaces (part of the 16th livraison published on 13 July 1822), Férussac (1822: 8) also considered two genera: the "premier genre," i.e., the first genus ever described, Buchannan’s Onchidium , and another genus, unnamed, with Onchidium peronii as type.

Onchis is not etymologically rigorous. The latinization of ὂγκος is oncos or oncus, as in the English word oncology. The Greek letter κ is “c” in Latin, while χ becomes “ch.” That Férussac used onchis instead of oncos is not surprising, as naturalists often took liberties with the latinization of Greek words. A famous example being the word taxonomy, created as taxonomie by De Candolle (1813: 19) from the Greek words taxis (arrangement, order) and nomos (law, rule): taxis should have stayed as taxi - to form taxinomie, taxinomy, exactly like in the English word taxidermy (from taxis and dermis, skin). However, the Code does not require taxon names to be etymologically correct. Therefore, the intentional spelling change of Onchis to Oncus by Agassiz (1846: 259; 1848: 748) is an unjustified emendation because Onchis is not the result of "inadvertent error, such as a lapsus calami or a copyist’s or printer’s error" ( ICZN 1999: Article 32.5.1) and therefore Onchis must not be corrected. The emendation of Onchidium into Oncidium by Agassiz (1846: 259; 1848: 748) also is unjustified for the same reason.

The generic name Peronia first appeared in two different venues, both published by Fleming (1822). One venue is Fleming’s (1822a: 574) article " Mollusca " in the fifth volume (second part) of the Supplement to the fourth, fifth, and sixth editions of the Encyclopædia Britannica published in May 1822 (as clearly indicated in a memorandum at the end of the sixth volume of the Supplement), even though the Supplement was only completed in 1824 (date on the title page). The other venue is Fleming’s (1822b: 463) Philosophy of Zoology which, according to Feuer and Smith (1972: 55), was not published earlier than May 1822 but no later than June 1822. The mention of Peronia in the Supplement is considered here to be the earliest one because it was published in May 1822.

Peronia Fleming, 1822a is an objective junior synonym of Onchis Férussac, 1822, because Férussac’s Onchis was published prior to Fleming’s Peronia and both generic names share the same type species ( Onchidium peronii ). However, to the best of our knowledge, Onchis has only been used twice in a binomen, and both times before 1899: by Stimpson (1855) for Onchis fruticosa , a species name that has remained unnoticed until now, and by Mörch (1863) for Onchis (Peronella) armadilla Mörch, 1863, i.e., Onchidella armadilla ( Mörch, 1863). Reversal of precedence applies here ( ICZN 1999: Article 23.9). Onchis , the senior synonym, "has not been used as a valid name after 1899" ( ICZN 1999: Article 23.9.1.1) and Peronia , the junior synonym, "has been used for a particular taxon, as its presumed valid name, in at least 25 works, published by at least 10 authors in the immediately preceding 50 years and encompassing a span of not less than 10 years." ( ICZN 1999: Article 23.9.1.2) A chronological list of 25 works meeting the criteria of ICZN Article 23.9.1.2 is provided here, all of which mentioning Peronia , Peronia verruculata , or Peronia peronii as valid names: Marcus and Marcus (1970), Starobogatov (1976), Britton (1984), Biskupiak and Ireland (1985), Faulkner (1987), Pietra (1990), Arimoto et al. (1993), Davies-Coleman and Garson (1998), Pietra (2002), Nakaoka et al. (2006), Carbone et al. (2009), Morrisey et al. (2010), Dayrat et al. (2011), White et al. (2011), Gaitán-Espitia et al. (2013), Mandal and Harkantra (2013), Sun et al. (2014), Bitaab et al. (2015), Harasewych et al. (2015), Liu et al. (2015), Wardiatno et al. (2015), Sun et al. (2016), Santhosh Kumar et al. (2016), Solanki et al. (2017), Xu et al. (2018). Onchis Férussac, 1822, objective senior synonym, is regarded as a nomen oblitum, and Peronia Fleming, 1822a, objective junior synonym, is regarded as a nomen protectum ( ICZN 1999: Article 23.9.1.2).

Fleming (1822a: 571, 574) classified Onchidium (with only the type species O. typhae ) in a group of slugs that "reside constantly on the land," and transferred O. peronii to Peronia , a genus for marine slugs that have "their residence constantly in water" and look like Onchidium . However, Fleming (1822a: 574) expressed doubts that Peronia slugs are air-breathing, as Cuvier (1804) claimed in the original description of O. peronii :

"This genus, which we have named in honor of M. Peron, was referred by Cuvier to the Onchidium of Buchanan ( …) and the species termed O. Peronii . It was found creeping upon marine rocks, under water, at the Mauritius, by M. Peron. M. Cuvier conjectures that it breathes free air, and has accordingly inserted it among the Pulmones aquatique [ Pulmonés aquatiques, i.e., aquatic pulmonates]. Some doubts, however, may reasonably be entertained about the truth of this supposition. It would certainly be an unexpected occurrence to find a marine gasteropodous mollusca obliged to come to the surface at intervals to respire. It will probably be found that it is truly branchiferous."

It was Audouin (1826) who demonstrated later that both Cuvier and Fleming were correct because Peronia peronii can breathe through both its pulmonary cavity and dorsal gills.

Blainville (1824: 280) created the generic name Peronia without being aware that Fleming (1822a, b) had already created exactly the same name two years before. Indeed, that Blainville (1824: 258) wrote "our genus Péronie " clearly suggests that he thought he was the author of Peronia . Also, most past authors attributed the authorship of Peronia to Blainville instead of Fleming (e.g., Stoliczka 1869: 100; Plate 1893: 102; Labbé 1934a: 189). Peronia Blainville, 1824 is a junior homonym of Peronia Fleming, 1822a and thus cannot be used as a valid name ( ICZN 1999: Article 52.2). However, Peronia Blainville is also a junior objective synonym of Peronia Fleming, because they "both denote nominal taxa with name-bearing types whose own names are themselves objectively synonymous." ( ICZN 1999: "objective synonym" in the glossary) Indeed, O. peronii , the type species of Peronia Fleming, and P. mauritiana , the type species of Peronia Blainville are objective synonyms because they share the same lectotype, i.e., the specimen from Mauritius which Cuvier (1804: pl. 6) illustrated (see below, the comments on the type material of O. peronii and P. mauritiana ).

When he created the generic name Peronia , Blainville (1824: 280, 281) cited only one species name, Peronia mauritiana , a junior objective synonym of Onchidium peronii . Blainville (1824: 281) also claimed that he knew four or five other species of marine onchidiids from the southern hemisphere, without naming them, but Blainville (1826: 523) listed them two years later (Table 1 ): Peronia laevis , a junior objective synonym of Marmaronchis vaigiensis ; Peronia semituberculata , a junior objective synonym of Onchidium planatum , itself a nomen dubium which may or may not refer to an onchidiid species; Peronia oniscoides , which all authors ignored except for Labbé (1934a: 243) and which clearly does not refer to a Peronia species (see general discussion). In addition, Blainville (1826: 523) also pointed out that Onchidium celticum , a name which Cuvier used for small marine slugs from the coast of Brittany, France, could also refer to a Peronia ; Onchidium celticum remained a nomen nudum until 1832, when it was described by Audouin and Milne-Edwards (1832: 118).

Like Cuvier (1804), Férussac (1822), and Fleming (1822a, b), Blainville (1824) did not mention the existence of dorsal gills. Dorsal gills were first described by Audouin (1826: 18-20) in the explanation of a plate by Savigny (1817: pl. 2) from the Description de l’Egypte. Savigny’s (1817: pl. 2, figs 3.1-3.8) plate displays eight drawings for two onchidiid slugs from the Red Sea, with one of them clearly representing a dorsal gill ( Savigny 1817: pl. 2, fig. 3.5). According to Audouin (1826: 19), it was Cuvier himself who identified those two slugs as Onchidium peronii , although Cuvier (1830) later changed his mind and created the new name Onchidium verruculatum for them. More importantly, Audouin (1826: 19) described in great detail the "small vascular branches" at the posterior end of the dorsum, or “tubercles” that work as "true gills." And Audouin (1826: 19) even made this clever statement:

"The Onchidie thus would have at the same time a pulmonary apparatus and a branchial apparatus; and that structure is in perfect agreement with what we know of the habits of that mollusk: Péron says that it is aquatic; on the contrary M. Cuvier, without the authority of this observer, would have believed it to be terrestrial. (...) We think that the Onchidie, at least the species illustrated here, enjoys the capacity to breathe under water thanks to the help of those ramified tubercles which cover the posterior end of its body, without the necessity of coming up to the surface; which is relatively difficult for an animal that slowly crawls at the bottom underwater. As for the pulmonary opening, it indicates that the onchidie breathes air as well; and we must suppose that several times in its life it finds itself in the condition to do so."

Audouin supposedly assumed that those slugs were truly aquatic.

Because Peronia was originally used as a genus for all marine onchidiids by both Fleming (1822a, b) and Blainville (1824, 1826), several Peronia species names already existed by 1830: Peronia mauritiana , P. peronii , P. oniscoides , P. semituberculata , and P. laevis (see above). Of those names, only the two objective synonyms P. mauritiana and P. peronii refer to true Peronia slugs, i.e., slugs with dorsal gills (Table 1 ). Cuvier (1830: 46) did not see the need for a genus assignment for marine onchidiid species and still only recognized Onchidium , but other naturalists started transferring species names from Onchidium to Peronia . Lesson (1833: pl. 19) transferred his own Onchidium ferrugineum Lesson, 1831a to Peronia , and clearly specified that he agreed with Blainville that marine onchidiids should be classified in a distinct genus. Dorsal gills are very clearly described by Lesson (1831a: 128-130; 1831b: 300-302; 1832: 36-37, fig. 32; 1833: pl. 19) in O. ferrugineum , but they were not the reason why he transferred it from Onchidium to Peronia . Shortly after that, Oken (1834a) also transferred six Onchidium species names by Quoy and Gaimard (1832-1833) to Peronia ( P. cinerea , P. incisa , P. nigricans , P. patelloides , P. punctata , and P. tongana ), with no justification but most likely because he also adopted the idea that marine onchidiids should not be classified in Onchidium .

The name Eudrastus was created by Gistel (1848: x), as a replacement name for " Peronia (Quoy, Isis 1834. 287.)." Gistel refers here to a report ( Oken 1834a: 283-310) on Quoy and Gaimard’s (1832-1833) contribution to the Voyage de découvertes de l’Astrolabe published in Isis, the encyclopedic journal edited by Lorenz Oken from 1817 to 1848. This report was most likely written by Oken himself, as was often the case ( Kertesz 1986), which would explain that the six onchidiid specific names mentioned ( tongana , incisa , patelloides , nigricans , punctata , cinerea ) are combined with Peronia instead of Onchidium , the generic name originally used by Quoy and Gaimard (1832-1833). Regardless of who authored that Isis report, Gistel (1848) did create the new generic name Eudrastus for those six species. Baker (1938: 86) subsequently designated Onchidium tonganum Quoy & Gaimard, 1832 ( Peronia tongana in Isis), as the type species of Eudrastus . Onchidium tonganum is regarded here as a junior subjective synonym of Peronia peronii , so Eudrastus is a junior subjective synonym of Peronia . Britton (1984: 182-183) suggested that Eudrastus should be regarded as a junior synonym of Peronia because it seemed to be based on "unimportant characters."

John Edwards Gray (1847: 179) attributed the authorship of Peronia to Blainville (with an erroneous date of 1825) but, most importantly, gave its modern definition to Peronia by restricting it to six species of slugs with "radiating processes" on the back (JE Gray 1850: 117): P. alderi , P. ferruginea , P. mauritiana , P. peronii , P. punctata , and P. tongana . All those names refer to true Peronia slugs with dorsal gills. JE Gray (1850: 117) restricted Onchidium to Buchannan’s O. typhae and included all the other marine species without dorsal gills in a new genus Onchidella .

JE Gray’s (1850) clarity only lasted for a few years. Indeed, Adams and Adams (1855: 234) pointed out that Peronia slugs differ from Onchidium and Onchidella because of "arbusculiform and other appendages of the mantle, which have sometimes been mistaken for gills." Because they did not believe that gills were distinct from other dorsal papillae, Adams and Adams (1855: 234) classified in Peronia some names that belong to both Peronia ( P. ferruginea , P. mauritiana , P. peronii , P. punctata , P. tongana ) and to Onchidella ( O. celtica , O. indolens , O. marginata , and O. parthenopeia ).

JE Gray’s (1850) classification was adopted by Keferstein (1865a) but, until Labbé’s (1934a) work, all authors have ignored the genus Peronia and simply used the genus Onchidium for slugs with and without dorsal gills ( Stoliczka 1869; Semper 1880-1885; Plate 1893; Bretnall 1919; Hoffmann 1928). Stoliczka (1869: 100-102), who was the first one to re-examine live slugs of O. typhae since Buchannan (1800), firmly argued that slugs with "dorsal tufts" were anatomically so similar to Onchidium and Onchidella that only one name, Onchidium , was needed. Stoliczka (1869: 98) also clarified that O. typhae is not a terrestrial species but that, instead, it lives in "damp places, generally close to tanks or ditches, especially those which are supplied during high tide with brackish water."

Stoliczka’s (1869) strong influence can be seen in Semper’s (1880-1885) study of the onchidiids from the Philippines (and other parts of the Indo-West Pacific) in which all onchidiids are in Onchidium , with the exception of a single species in his new genus Onchidina Semper, 1882; for a collation of Semper’s work, see Johnson (1969). Plate (1893) adopted a classification with five genera, but the four species of slugs with dorsal gills recognized by Plate are classified in Onchidium with thirteen species of slugs without dorsal gills. Hoffmann (1928) adopted a classification with six genera, six species of slugs with dorsal gills being classified in Onchidium with 34 species without dorsal gills.

Then, suddenly, in 1934, the number of onchidiid taxon names for slugs with dorsal gills dramatically increased. Based on the onchidiid collection at the Paris Museum, Labbé (1934a) created fourteen new species-group names for slugs with dorsal gills (all but one name are species names) and four new generic names: Lessonia (later replaced by Lessonina ), Paraperonia , Quoya , and Scaphis . Below, the nomenclatural status of Labbé’s generic names is justified first (they all are junior synonyms of Peronia ), followed by opinions in the secondary literature.

The generic name Paraperonia was created by Labbé (1934a: 196) for four species similar to Peronia but with intestinal loops of type V (instead of type I). The type species is Paraperonia gondwanae Labbé, 1934a, by subsequent designation ( Starobogatov 1976: 211). Labbé’s description of Paraperonia gondwanae was based on 38 individuals with intestinal loops of types I and V which belong to different species. The application of the name P. gondwanae is clarified through the designation of a lectotype (see P. verruculata ): Paraperonia gondwanae is a junior synonym of Peronia verruculata , and Paraperonia is a junior synonym of Peronia .

The generic name Scaphis was created by Labbé (1934a: 203) for nine species similar to Peronia but supposedly with an oblique, almost vertical hyponotum. The type species is Onchidium astridae Labbé, 1934b, by subsequent designation ( Starobogatov 1976: 211). Onchidium astridae is a junior synonym of Peronia verruculata , and Scaphis is a junior synonym of Peronia .

Lessonia Labbé, 1934a is objectively invalid because it is the junior homonym of Lessonia Swainson, 1832 [ Aves]. Starobogatov (1976: 211) replaced it by Lessonina . Labbé (1934a: 213-216, figs 48-50) described Lessonia based on a single species, Onchidium ferrugineum Lesson, 1831a, of which he examined no other material than the four syntypes (MNHN-IM-2000-22951). The examination of the three remaining syntypes (one syntype was lost by or after Labbé) revealed that the lectotype ( Goulding et al. 2018b: 75) belongs to a Peronia species and that the two paralectotypes belong to Wallaconchis ater (Lesson, 1831a). Both Lesson’s original description of Onchidium ferrugineum and Labbé’s re-description of Lessonina ferruginea are a confusing combination of traits that characterize species from two distinct genera. For instance, the dorsal gills mentioned by both authors, are characteristic of Peronia , while the absence of an accessory penial gland mentioned by Labbé (even though there is a penial gland in the lectotype) is characteristic of Wallaconchis . Thanks to the designation of a lectotype with dorsal gills, the name Onchidium ferrugineum clearly applies to a Peronia species and Lessonina becomes a junior synonym of Peronia .

Starobogatov (1976: 211) created Quoyella as a replacement name of Quoya Labbé, 1934a, which he treated as a junior homonym of " Quoya Deshayes, 1843" [ Mollusca, Gastropoda, Planaxidae ]. In the second edition of Lamarck’s Histoire naturelle des animaux sans vertèbres, Deshayes indicates that he originally thought of creating a new genus Quoya but that, after all, he decided not to ( Deshayes and Milne-Edwards 1845: 236). Deshayes still used the binomen " Planaxis decollata Quoy" ( Deshayes and Milne-Edwards 1845: 238). However, in the Explication des planches of his Traité élémentaire de conchyliologie, Deshayes (1853: 50) used Quoya for two valid species names: Quoya decollata and Quoya grateloupi . Regardless, according to Gray (1847: 138), the generic name Quoya by Deshayes is an incorrect subsequent spelling of his Quoyia JE Gray, 1839. As an incorrect subsequent spelling, Quoya Deshayes is not available ( ICZN 1999: Article 33.3) and, as a result, Quoyella is an unnecessary replacement name. Ironically, Gray (1847: 138) indicated that he originally found the generic name Quoyia in a manuscript by Deshayes in 1830 (" Quoyia , Desh. MSS. 1830; Gray, 1839 (...) Quoya , Desh. 1843"). According to Baker (1938: 87), Quoya Agassiz, 1862 [ Coelenterata] is another homonym of Quoya Labbé, 1834a. However, the spelling of that generic name is not Quoya but Quoyia (Agassiz, 1862: 173). So, Quoyia Agassiz, 1862 is a junior homonym of Quoyia Gray, 1839, but Quoya Labbé, 1934a is not a junior homonym of Quoyia . Quoya indica Labbé, 1934a, type species of Quoya by monotypy, is regarded here as a nomen dubium even though it applies to a species with dorsal gills and thus belongs to Peronia (see general discussion).

Nothing is ever simple in onchidiid taxonomy. Indeed, Labbé (1935a, b) also described what he called “microgills” in Elophilus Labbé, 1935a, a name preoccupied by Elophilus Meigen, 1803 ( Diptera ) and replaced by Labbella Starobogatov, 1970. Labbé’s (1935a, b) microgills consolidated the old idea of a gradual continuum between regular dorsal papillae and dorsal gills. So, for instance, Marcus and Marcus (1960: 875) argued that one cannot say for sure whether a papilla is a dorsal gill or not. However, Dayrat et al. (2016, 2019d) demonstrated that there are no gills at all (not even microgills) on the notum of the type material of the type species of Labbella which actually belongs to Onchidium stuxbergi (Westerlund, 1883). Labbella is a junior synonym of Onchidium . Contrary to regular papillae, dorsal gills are distinctively branched, which is striking if specimens are fully relaxed before preservation but otherwise difficult to see. Finally, note that Labbé (1935b: 320) claimed that he observed rudimentary eyes on dorsal gills, which, to our knowledge, has never been confirmed.

Labbé (1934a: 187, 188) rightly recognized the importance of dorsal gills for classification and he separated all five genera of slugs with dorsal gills from all other onchidiids. According to Labbé, onchidiids deserved their own order, the Silicodermatae, composed of two suborders: Dendrobranchiatae (onchidiids with dorsal gills) and Abranchiatae (onchidiids without dorsal gills). Our phylogenetic analyses clearly demonstrate that all species of slugs with dorsal gills belong to a single clade, and that only one generic name ( Peronia ) is necessary (Figs 2 - 4 ). However, the species of slugs with no dorsal gills do not form a natural group (Figs 2 - 4 ). In other words, the absence of dorsal gills is a plesiomorphic trait for the onchidiids and the presence of dorsal gills is a synapomorphy for the genus Peronia .

Labbé’s (1934a: 187) distinction between the tribes Peroniidae ( Peronia and Paraperonia ) and Scaphidae ( Scaphis , Lessonina , Quoya ) based on the orientation of the hyponotum (horizontal versus oblique) is meaningless. This trait obviously varies depending on preservation, and Labbé exclusively studied preserved material from the collections of the MNHN without access to live animals.

Labbé’s (1934a: 187) distinction between Peronia and Paraperonia based on the intestinal types (type I in Peronia and type V in Paraperonia ) is unwarranted because Peronia species with intestinal loops of type V are not more closely-related to each other (Table 4 , Figs 2 - 4 ). Also, Labbé often made mistakes with respect to intestinal types: for instance, the type material of Paraperonia gondwanae includes individuals with loops of both types I and V, even though Labbé described it as a species with loops of type V. Labbé asserted that the position of the pneumostome and the size of the muscular sac differ between Peronia and Paraperonia . However, the position of the pneumostome varies between individuals and is not consistently on the right side of the median axis in species he classified as Paraperonia .

Labbé’s (1934a: 187) distinction between Scaphis , Quoya , and Lessonina , is also unwarranted. Again, the position of the pneumostome (on the right of a median line in Scaphis according to Labbé) varies between individuals. Labbé’s (1934a) re-description of Lessonina ferruginea (the type species of Lessonina , by monotypy) was based on individuals of two different species (see above). The male opening of the lectotype, which bears dorsal gills, is on the left of the right ocular tentacle, exactly as in all Peronia species, while the male opening of the two paralectotypes, which belong to Wallaconchis ater , is under the right ocular tentacle (Goulding et al. 2018: 75). Labbé (1934a: 216, fig. 51) described a double male opening in Quoya indica (the openings of the penis and of the accessory penial gland being supposedly separated), but this could not be confirmed in the type material. Regardless, male openings occasionally appear separated due to preservation (when the vestibule is everted) and that is by no means a trait of generic value.

Authors completely rejected Labbé’s (1934a) idea that the presence or absence of dorsal gills could be of any use in onchidiid classification (e.g., Marcus and Marcus 1960; Starobogatov 1976). Britton (1984: 180) even asserted that "the division of the group into two subordinate taxa based on this character is no longer admissible." As for the status of Labbé’s (1934a, 1935a) generic names for slugs with dorsal gills, authors were not in agreement. Marcus and Marcus (1970: 213) regarded Peronia and Paraperonia "at most as subgenera." Starobogatov (1976) regarded all names as valid: Lessonina , Paraperonia , Peronia , Quoyella (unnecessary replacement name for Quoya ), Scaphis , and Labbella (supposedly with micro-gills). Britton (1984: 182-183) suggested that Paraperonia , Eudrastus and Scaphis should be regarded as junior synonyms of Peronia because they seemed to be based on "unimportant characters," but treated Labbella (supposedly with micro-gills), Lessonina , and Quoyella (for Quoya ) as valid. In a recent review of the application of onchidiid generic names, Dayrat et al. (2017: 1861) made it clear that all slugs with dorsal gills belong to one clade and that Eudrastus , Lessonina , Onchis , Paraperonia , Peronia , Quoyella (for Quoya ), and Scaphis all refer to that clade. Note that the application of Lessonina was fully clarified when a lectotype was designated for its type species Onchidium ferrugineum (Goulding et al. 2018: 75).

Figures

Figure 2. Phylogenetic relationships between Peronia species based on concatenated mitochondrial COI and 16 S DNA sequences. Numbers by the branches are the bootstrap values (maximum likelihood analysis) and the posterior probabilities (Bayesian analysis). Only the values> 50 % (ML) and> 0.9 (Bayesian) are indicated. Numbers for each individual correspond to unique identifiers for DNA extraction. Information on specimens can be found in the lists of material examined and in Table 2. The color used for each species or mitochondrial unit is the same as the color used in Figs 3 - 6.

Figure 4. Maximum parsimony consensus tree within Peronia based on concatenated nuclear ITS 2 and 28 S sequences. Numbers by the branches are the bootstrap values. Only the values> 50 % are indicated. Numbers for each individual correspond to unique identifiers for DNA extraction. Information on specimens can be found in the lists of material examined and in Table 2. The color used for each species or mitochondrial unit is the same as the color used in Figs 2, 3, 5, 6.