Tomostoma angusta, Symonds, Malcolm Francis & Pacaud, Jean-Michel, 2010

Symonds, Malcolm Francis & Pacaud, Jean-Michel, 2010, New species of Neritidae (Neritimorpha) from the Ypresian and Bartonian of the Paris and Basse-Loire Basins, France, Zootaxa 2606, pp. 55-68 : 56-58

Tomostoma angusta sp. nov.

( Fig. 1 a–d)

Type material. Holotype MNHN A31487 (Pacaud leg., Fig. 1 a–d); 6 paratypes MNHN A31488 (Pacaud leg.), 30 paratypes (Pons coll.). All from type locality.

Stratum typicum. Late Eocene, Bartonian, “Marinesian”.

Locus typicus. Le Quoniam, Haravilliers, Val d’Oise, France. 49˚11ˏ2.68˝ N 2˚2ˏ17.53˝ E.

Derivatio nominis. The name is derived from Latin angustus, referring to the narrow shape of the shell.

Diagnosis. A rather small, narrow Tomostoma , capuliform, posterior and anterior margins of teleoconch ovate, not pointed, septum edge strongly concave in centre.

Description. Protoconch, though rather worn in specimens available, prominent, approximately 0.2 mm wide, smooth, obovate. Teleoconch capuliform, apex above posterior end in holotype, slightly anterior to it in most paratypes. Anterior and posterior margins ovate with posterior usually slightly narrower than anterior. Dorsal surface strongly convex, sloping steeply upwards to elevated posterior end, smooth apart from fine growth lines. No colour pattern visible on only known specimens. Aperture relatively large, semicircular. Inner lip protruded as broad septum covering more than half of base of shell, surface slightly concave, usually smooth but holotype has 3 faint grooves about 0.2 mm long perpendicular to septum edge coinciding with spaces between teeth on central part of septum. Septum edge strongly concave in centre with 1 to 5 small, prominent or indistinct teeth bordered on adapical side by larger tooth, which continues for short distance within aperture as low ridge on inner side of septum, followed by further concavity, short but quite deep. On abapical side of central concavity one broad, rather rounded tooth. Outer lip broad, smooth with very slight ridge running from one end of septum, in arc on inside of outer lip, to other end of septum. Small ridge below each end of septum, adapical larger, below septum edge, abapical narrower, lower, set further back.

Size. The holotype is 5.2 mm long and 2.8 mm wide.

Remarks. Three somewhat similar taxa occur in the Eocene of the Paris Basin and Cotentin: Tomostoma altavillensis altavillensis ( Defrance, 1818) , Tomostoma altavillensis neritoides (Deshayes, 1824) and Tomostoma altavillensis rostratum Cossmann, 1888 ( Le Renard & Pacaud, 1995) . Of the three, T. altavillensis neritoides is closest in appearance to T. angusta with its anterior and posterior margins oval, not pointed as in the other two taxa. Tomostoma angusta can be distinguished from T. altavillensis neritoides by the concave centre to its septum edge, the smaller, more regular teeth in the centre and its narrower form. The average length to width ratio for T. angusta is 2.3:1 (from the 37 specimens in the Pacaud and Pons collections) compared with 1.6:1 for T. altavillensis neritoides (from 30 specimens from the Lutetian of Chaussy, Val d’Oise, France). A t-test applied to the length to width ratio of these specimens results in a probability of <0.0001.

Range and distribution. Tomostoma angusta is known only from the type locality.

Figures

FIGURES 1 – 4. 1. Tomostoma angusta sp. nov., holotype, a. apertural, b. abapertural, c-d. side views. Length 5.2 mm, width 2.8 mm. Bartonian, Le Quoniam, Haravilliers, Val d’Oise, France. MNHN A 31487 (J-M Pacaud leg.). 2. Nerita gouetensis sp. nov., holotype, a. apertural, b. abapertural, c. apical views. Height 4 mm, width 4 mm. Bartonian, Bois- Gouët, Saffré, Loire-Atlantique, France. MNHN A 31489 (J-M Pacaud leg.). 3. Clithon barbei sp. nov., holotype, a. apertural, b. abapertural, c. apical views. Height 3.2 mm, width 3.2 mm. Ypresian, Pourcy, Marne, France. NHM PI TG 26448 (M. F. Symonds coll.). 4. Neritodryas guillioui sp. nov., holotype, a. apertural, b. abapertural, c. apical views. Height 15 mm, width 16 mm. Ypresian, Pourcy, Marne, France. MNHN A 31490 (Faullummel coll.).