Wollastonia ripkeni De Mattia & Groh

Mattia, Willy De, Neiber, Marco T. & Groh, Klaus, 2018, Revision of the genus-group Hystricella R. T. Lowe, 1855 from Porto Santo (Madeira Archipelago), with descriptions of new recent and fossil taxa (Gastropoda, Helicoidea, Geomitridae), ZooKeys 732, pp. 1-125 : 33-37

† Wollastonia ripkeni De Mattia & Groh sp. n. Figs 122-124, 125

Type material.

SMF 348929, holotype, from loc. typ., leg. W. De Mattia & J. Macor, May 2015, NMWC Z.2016.013.00008, 2 PT (paratypes), NHMW 112144/1 PT, CKG, 2 PT, CWDM, 11 PT, from loc. typ., leg. W. De Mattia & J. Macor, May 2015.

Locus typicus.

Porto Santo, excavated mud walls behind the cart speedway E of the new harbour of Porto Santo, 33°03'48"N / 16°18'22"W, 30 m.

Diagnosis.

Shell large for the genus, solid and conical. Whorls not rounded, not vertical but somewhat slanting. Surface of the shell covered by small, and relatively densely set tubercles. Body whorl bicarinated, with upper keel weaker than the lower one. Remaining teleoconch whorls only with the upper keel visible. Umbilicus narrow. Last whorl descending toward the aperture. Aperture oval with continuous peristome.

Description of the holotype.

Shell large for the genus, with 6⅔ regularly growing whorls, the protoconch with 1.85 whorls. The form of the shell is conical, the convex teleoconch whorls showing two rounded keels, the upper one less developed than the lower one. The last whorl measures 66%, the penultimate whorl 12% of the total shell height. The lower ¾ of the body whorl are beneath the lower peripheral keel, which is set off by a constriction above and below the keel; outline of body whorl below the lower constriction regularly convex in frontal view. The two keels of the body whorl are placed in the upper 1∕11 and ¼ of its total height. The suture between the whorls is simple, not sunken. The aperture, which is inclined to the vertical axis of the shell in an angle of 50° and is descending in the last 5% of the last whorl in an angle of 35° to the horizontal axis, has an elliptic form; its width is 46%, of the total shell width, its height 31% of the total shell height. It has a slightly reflected lip, which is completely detached from the body whorl. The eccentric umbilicus, which measures 10% of the total shell width, is in the last whorls circular and completely closed in the earlier whorls. The protoconch is smooth, the teleoconch shows a number of oblique radial ribs, 13 in the penultimate quadrant of the body-whorl and is additionally covered by numerous rounded tubercles. The number of tubercles in the standard-quadrate of the base is 89. There are no traces of colouration (Figs 122-124).

Variation of the paratypes.

Although the shell size is slightly variable, its shape is remarkably stable in all specimens.

Measurements.

D 8.0 ± 0.4 mm (range 7.2-8.6 mm); H 6.2 ± 0.4 mm (range 5.4-6.8 mm); FW 4.1 ± 0.2 mm; PA 42 ± 4.1°; DU 0.9 ± 0.08 mm; NT 89 ± 23; NW 6.7 ± 0.2 (n = 16). Ratio D/H 1.3; ratio FW/H 0.7.

Distribution.

Known only from the locus typicus. The species was hitherto found only along a 50 m stretch of mud deposits close to the cart speedway. Additional deposits in the vicinity could not be checked for the presence of the species because of difficult accessibility of the steep slopes and associated risks of stone fall and landslides. The currently known distribution is shown in Fig. 125.

Etymology.

Named for the Dutch malacologist Theodor ( “Theo”) J. E. Ripken from Delft, The Netherlands, to honour his valuable contributions to the malacofauna of the Macaronesian Islands.

Comparison and comments.

Wollastonia ripkeni sp. n. can be confused on first glance with other large-sized species such as H. echinoderma , W. falknerorum sp. n. or W. vermetiformis . From the first it is separated by the development of two distinct keels, from the second by a much finer sculpture, a narrower umbilicus and narrower aperture. From the quite similar H. vermetiformis it differs by a much more solid shell, a much higher last whorl, a larger aperture, finer granulation and coarser ribbing of the body whorl. From similar sized W. subcarinulata and W. inexpectata sp. n. it is separated by a coarser granulation and the presence of two keels.

Taxonomic remarks.

Wollastonia ripkeni sp. n. is included in the genus Wollastonia because it is similar to W. oxytropis in size and surface sculpture, considerably differing from Hystricella s. str. where the tubercles are bigger and less densely set. We are well aware that, due to the lack of anatomical and molecular data, the generic placement of this subfossil species is exclusively based upon shell features, with all the taxonomic limits that this might implicate.

Status and conservation.

Extinct before the islands’ scientific exploration in the 19th century, possibly already before human settlement.