802 



DIVISIONS OF THE GASTROPODA. 



shells," in which the shell is ventricose, with a very large body- 

 whorl and a deeply sunk spire. The fossil types are principally 

 found in the Tertiary deposits. The Recent and Tertiary genus 

 Scaphander is closely related to Bulla. Lastly, in Cylichna the 

 shell is solid, and cylindrical in form, with a narrow aperture and a 

 thickened columella. The species of this genus are numerous, and 

 range from the Trias to the present day. 



Family 3. Aplysiad^e. — In this family there is a thin horny 

 shell, which is concealed by the reflection of the mantle over the 

 back and sides of the animal. Two fossil species of Aplysia are 

 stated to occur in the Pliocene deposits of Sicily. 



Family 4. Pleurobranchid^e. — In this family the shell is 

 limpet-like, or concealed, or may be wanting. The genus Umbrella 

 appears to be represented in the Pliocene deposits, and dubious 

 Secondary forms of the same have also been described. 



Order III. Heteropoda. 



The Gastropods included in the order of the Heteropoda or 

 Nudeobranchiata differ from the typical members of the class in 

 being organised to lead an existence in the open ocean, locomotion 

 being effected by a Jin-like tail, or by a fan-shaped vertically-flattened 

 ventral fin. They are found swimming at or near the surface of the 

 ocean ; and the body may be completely protected by a shell, within 

 which the animal can retire, and which can be closed by an oper- 



Fig. 



717.— Heteropoda. Carinaria cymbium. p, Proboscis; t, Tentacles; b, Branchiae; 

 s, Shell ; /, Foot ; d, Disc. (After Woodward.) 



culum. In other cases, as in Carinaria (fig. 717), the body is 

 large, and there is only a small shell protecting the gills and heart. 

 In other cases, again, the shell is completely wanting. The order 

 is divided into the two families of the Firolidce and Atlantidcz, both 

 of which comprise transparent delicate animals, which are either 

 wholly naked or possess a shell of extreme tenuity and fragility. 



