534 



MOLLUSCA 



PHYLUM VI 



Platychilina Koken {Fossariopsis Laube). Spire depressed, straight ; last whorl 

 large, surface rough, tuberculose. Inner lip even, with simple margin. Trias. P. 

 pustulosus (Miinst.). 



Delphimdopsis Laube. Like the last, but spire composed of loosely connected 

 whorls. Sutures deep. Body whorl with nodose longitudinal keels. Inner lip even, 

 with sharp margin. Trias. D. binodosa (Miinst.). 



Neritoiosis Grat. (Figs. 901, 902). Spire depressed, Ijody whorl very large. 

 Surface Avith sjjiral and transverse ribs or nodes, often cancellated. Inner lip 

 thickened, with broad, angular emargination in the middle. Trias to Kecent. 



Family 14. Neritidae Lamarck. 



Shell semi-globose, without 'umbilicus or nacreous layer. Spire very short, someivhat 

 lateral; whorls rctpidly broadening, the last very large, and earlier ones resorbed 

 internally. Aperture semicircular ; margin of the flattened or calloused inner lip often 

 with teeth. Operculum calcareous, with a lateral spiral nucleus, and a process for 

 ■muscle attachment on the inner side. Trias to Recent. 



The Neritidae are partly marine, and partly fresh-water inhabitants. The former 

 live usually in the vicinity of the coast, the latter often in brackish water. Since the 

 earlier whorls are internally resorbed, moulds of the interior reveal no trace of the spire. 

 This character, together with the form of the operculum, serves to distinguish the 

 family from the Naticopsidae, from which both it and the terrestrial Helicinidae 

 are probably descended. Fossil forms not infrequently retain traces of their former 

 coloration. 



Neritaria Koken {Protonerita Kittl). Spire acuminate, suture deep, surface 

 smootli. Outer lip sharj) ; inner lip callous, flattened. Resorption of the inner walls 

 incomplete. Trias. 



Nerita Linn. (Fig. 903). Thick, ovoid or semi-globose, imperforate. Surface 

 smooth or with sjjiral ribs. Inner lip callous, flattened, with a straight, often 

 denticulate border. Operculum sub-spiral. Trias (?) to Recent. 



(?) Oncochilus Petho (Fig. 904). Smooth ; inner liji arched, callous, bearing two or 

 three teeth on the margin or smooth ; outer lip sharp. Trias and Jura. 



Lissochilus Petho (Fig. 905) ; Neritodomus Morr. and Lye. ; Neritoma Morris. 

 Jura. Otostoma d'Ai'ch. ; Dejanirco Stol. Cretaceous. 



Velates Montf. (Fig. 906). Depressed conical, only the curved apex of the sjjire 

 A n c 



Flu. 903. 



A, Nerita laffoniMeriiLJi. Citharella Limestone ; Epfen- 

 hofen, near Sciiaflliaiisen, Switz(>rland. B, N. (jrannlosa 

 Desh. Eocene (Sables Moyens) ; Auvers, near Paris. C, 

 Operculum of a recent Nerita. 



Fii!. 904. 



Oncochilus chronKiticus Zittel. Upper 

 Tithonian ; Stramberg, Moravia. 



visible. Last whorl very large. Inner lip convex or straight, with denticulate margin. 

 Abundant in the European Eocene ; sometimes attaining a size of 10 or 12 cm. 



Neritina Lam. (Fig. 907). Small, semi-globose, lustrous, smootli or spiny, mostly 

 brilliantly coloured. Inner lip flattened, with sharp or finely toothed margin ; outer 

 lip sharp. Inhabits brackish or fresh water. Abundant in Tertiary and Recent. The 

 sujjposed Mesozoic forms belong i)rincipally to Nerita. 



