550 



MOLLUSCA 



PHYLUM VI 



transverse ribs and spiral striae. Aperture contracted, rounded, without canal, some- 

 times slightly separated off, and with continuous jieristome. Abundant in the Jura. 

 Bittium Leach (Fig. 980). Turreted, with granulated spiral ribs, 

 and numerous transverse costae. Aperture with short, straight canal ; 

 outer liji sharp. Jura to Eecent. Abundant in the Tertiary. 

 Triforis Deshayes ; Gerithiopsis Forbes. Tertiary and Recent. 



Eustoma Piette. Aperture with 

 long canal, which is often closed, 

 however, by margins of the inner 

 and oiiter lip. Inner lip callous 

 and strongly dilated ; outer lip 

 expanded. Jura. 



Cerithium Brug. (Figs. 981, 

 982). Turreted, imperforate, with- 

 out epidermis. Aperture oblong, 

 ovate, with l)ackwardly curved 

 canal ; outer liji often somewhat 

 reflected. Columella concave, fre- 

 quently with one or two folds. 

 Certain Tertiary species attain a length of half a metre {G. giganteum). Jura to 

 Recent ; maximum in Eocene. 



Subgenera : Vicarya d'Arcli. ; Clava Martyn (Fig. 982) ; Bcllardia Mayer, etc. 



Potamides Brongt. (Figs. 983, 984). Turreted, with epidermis ; aperture with 

 emargination or faint canal. Inhabits only brackish water or 

 estuaries. Cretaceous to Recent. 



Fig. 978. 



Filnila undulosa 

 Piette. Bathou- 

 ian ; Eparcy, 

 Aisne. 



Fig. 079. 



Exelissa strangu- 

 lata (d'Arch.). 

 B a-t li o II i a n ; 

 Eparcy, Aisne. 



Bittium pJicaJM7)i 

 Brng. Oligocene; 

 OriiKiy, near litampe.s, 

 France. 



Fig. 981. 



Cerithium serratum 

 Brug. Calcaire Gros- 

 sier ; Damery, near 

 Epernay. 



Pig. 982. 



Cerithium (Clava) 

 nudum, Lam. Eocene ; 

 Chaumont, near Paris. 



Fig. 983. 



Potamides (Tympano- 

 tumus) margaritaceum 

 Brocchi. Oligocene ; 

 Hackenheim, near Alzey. 



Fig. 984. 



Potamides (Lampaiiia) 

 plcurotomides Desh. 

 Middle Meere.ssand ; 

 Mortefontaine, Seine-et- 

 Oise. 



Subgenera : Tympanotovms Adam.s (Fig. 983) ; Pyrazus, Telcscopium Montf. 

 Swains. ; Lampania (Fig. 984) and Pyrenella Gray ; Sandhenjeria Bosq. 



Cerithidea 



Family 18. Aporrhaidae Philippi. 



Shell fusiform, turreted or conical ovate. Aperture produced anteriorly in a canal. 

 Outer lip expanded in a iving-like or digitiform fashion, or thickened. Operculum horny. 

 Marine. Jura to Recent ; maximum in Jura and Cretaceous. 



Alaria Morris and Lye. (Figs. 985-988). Shell turreted ; aperture with long or 

 sliort canal. Outer \\\\ not overriding the last whorl, digitated or winged. Spire 



