Tomostoma Deshayes, 1824

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 : 55

Genus Tomostoma Deshayes, 1824

Type species. By subsequent designation, Fischer (1885: 803): Pileolus neritoides, Deshayes, 1824 . Eocene, Europe.

Diagnosis. Capuliform, smooth, apex not terminal, aperture trapezoidal, inner lip with sinus, type species 7 to 9 mm in length. (Amended after Keen 1960).

Tomostoma differs from most other genera in Neritidae in being capuliform. For the differences from the other Tertiary capuliform genus see Cuisenerita gen. nov. below.

Remarks. Cossmann (1925) and Keen (1960) regarded Tomostoma as a subgenus of Pileolus G. B. Sowerby, 1823 . Bandel et al. (2000) erected a new family Pileolidae based on the genus Pileolus but did not comment on whether they regarded Tomostoma as a subgenus of Pileolus . They placed Pileolidae within Neritoidea Rafinesque, 1815 for which they gave the diagnosis: “In these cycloneritimorphs the internal shell walls are resorbed” (2000: 85). However Sowerby (1823: 443) stated that Pileolus has an “internal spire”. This is clearly shown by Pană (1998: Pl. 2, Fig. 13) in a section through the teleoconch of Pileolus . On this basis Pileolidae should not be included within Neritoidea. Symonds (2009) did not see a close similarity between Pileolus and Tomostoma as the protoconch of Pileolus is spherical and rather large in relation to the rest of the shell while the protoconch of Tomostoma is obovate and typical of those genera within the Neritidae that have a planktotrophic larval stage. The teleoconch of the former is distinctly patelliform unlike that of the latter, which is capuliform. The aperture of Pileolus is rather narrow and semilunar whilst in Tomostoma the aperture is much broader. Pileolus has a straight or slightly convex septum edge, which is smooth or regularly crenulated; in Tomostoma the septum edge is concave and the dentition distinct but very irregular. Additionally in Pileolus the septum is thickened to form a prominent basal plate, a feature not present in Tomostoma . Finally, as mentioned above, Pileolus has a columella while the internal walls of Tomostoma are entirely reabsorbed. As the morphology of both the protoconch and the teleoconch corresponds to Neritidae, Symonds (2009) regarded Tomostoma as being in Neritidae rather than Pileolidae and this is followed here. Woods & Saul (1986: 650) commented that “Deshayes is usually credited with proposing Tomostoma in 1824; but he did not do so.” They considered that it was not until 1864 that he validly proposed Tomostoma and that as a result Culana Gray, 1842 (type species Pileolus altavillensis Defrance, 1818 ) should have priority. Deshayes (1824a; 1824b) made it clear that he had intended to propose the name Tomostoma , but he did not do so at the time as he considered that it was synonymous with Pileolus , which had just been proposed by Sowerby (Sowerby, 1823). Later, Deshayes (1864: 25) considered that there were two distinct forms of Pileolus : those from the Mesozoic with a circular base and central apex and those from the Tertiary with an elliptical base and an apex towards the posterior end; to the latter group could be attached the name Tomostoma “que nous avons proposé autrefois”. Article 11.6.1 of the International Code of Zoological Nomenclature (the Code) provides that if a name first published as a junior synonym is subsequently used as a valid name before 1961, it dates from its first publication as a synonym. Accordingly Tomostoma was published in 1824 and Culana is here considered a subjective junior synonym.