420 MONOTOCARDIA TAENIOGLOSSA chap. 



nucleus. Cretaceous . Genera : Cassis (subg. Seinicassis 



and Cypraecassis), Morio ( == Cassiclaria), Oviiscia. 



Fam. 41, Colurribellinidae — Shell solid, ribbed, usually 

 cancellated, with an oblique posterior canal, columella callous, 

 more or less reflected. Genera : Colunibellina, Columlellaria, 

 Zittelia, Peter sia, Alariopsis (?). Secondary strata only. 



Fam. 42. Tritonidae. — Foot short, narrow; siphon short, not 



prominent ; radula allied to that of Cassididae ; shell thick, 



varicose ; outer lip inflected and thickened, canal long, perios- 



tracuni often thick and hairy, operculum 



corneous, nucleus terminal or sub-marginal. 



Cretaceous . Genera: Triton {Yig. 1^1, 



p. 275 ; subg., Upidromtis, Flesiotriton, Sim- 

 ptdum, Manularia, Argohuccinum) ; Perso7ia, 

 aperture toothed, narrow ; columella reflected 

 upon the last whorl ; Ranella, shell dorso- 

 ventrally compressed, generally with two con- 

 tinuous lateral varices, posterior canal present. 

 The position of the following four families 

 is doul)tful :^- 



Fam. 43. Oocorythidae. — Siphon short, 

 foot broad, eyes absent, radula taenioglossate ; 

 Fia. 278— Pirula Dus- shell buccinoid or cassidiform, operculum cor- 

 pines. X 4.' neous. Spiral. — Eecent. Single genus, Oocorys. 



Fam. 44. Subulitidae. — Shell elongate, 

 fusiform, smooth ; suture shallow, base truncate or rounded, 

 apertm-e channelled or notched. Ordovician to Trias. Genera : 

 Subulites, Fusispira, Fuchrysallis. 



Fam. 45. Seguenziidac. — Eadula taenioglossate, shell trochi- 

 form, aperture channelled, columella twisted, operculum multi- 



spiral, nucleus central. Pliocene . Single genus, Seguenzia. 



Fam. 46. Choristidae. — Anterior tentacles united by a frontal 

 veil, posterior simple ; eyes absent, foot with tentaculae before and 

 behinil ; three central teeth, outer marginal with a basal plate ; 

 shell helicoid, suture deep, peristome continuous, operculum 



corneous, paucispiral. Pliocene . Single genus, Choristcs. 



Section II. Heteropoda. — Foot fin-shaped, not flat. 

 The Heteropoda are free-swimming Mollusca, being, like the 

 Pteropoda, Gasteropoda modified to suit their pelagic environ- 

 ment. Their nervous system is streptoneurous, and they are 



