420 



MONOTOCARDI A — TAENIOGLOSSA 



CHAP. 



nucleus. Cretaceous 



Genera : Cassis (subg., Semicassis 



and Oypraecassis)^ Morio (^—Cassidarid)^ Oniscia. 



Fam. 41. CohwihelUnidae. — Shell solid, ribbed, usually can- 

 cellated, with an oblique posterior canal, columella callous, more 

 or less reflected. Genera : Columhellina^ Columhellaria., Zittelia^ 

 Pefersia, Alarioijsis (?). 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- 



tracum often thick and hairy, operculum 

 corneous, nucleus terminal or sub-marginal. 



Cretaceous . Genera : Triton (Fig. 191, 



p. 275 ; subg., Epidromus^ Plesiotriton, Sim- 

 jjulum, Ranularia^ Argohuccinum) ; Pey^sona^ 

 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 doubtful : — 



Fam. 43. Oocorythidae. — Siphon short, 

 foot broad, eyes absent, radula taenioglossate ; 

 shell buccinoid or cassidiform, operculum cor- 

 neous, spiral. Recent. Single genus, Oocorys. 

 Fam. 44. Siibulitidae. — Shell elongate, 

 suture shallow, base truncate or rounded, 

 aperture channelled or notched. Ordovician to Trias. Genera : 

 Suhulites^ Fusispira^ Eiiehrysallis. 



Fam. 45. Seguenziidae. — Radula 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 

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

 shell helicoid, suture deep, peristome continuous, operculum 



corneous, paucispiral. Pliocene . Single genus, Clioristes. 



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 



Fig. 218.—Pirula Bus 

 sumieri Val., Philip- 

 pines. X ^. 



fusiform, smooth ; 



