SHELLS. 229 



The shells of this genus are very simple in form; 

 they are all slightly rolled up, without being spiral. 



The only known species is the Bullcea aperta ; shell 

 somewhat rounded, pellucid, shghtly striated trans- 

 versely, and the aperture very large; it climbs 

 aquatic plants well, but swims badly. 



V. Gasteropoda Heteropoda. — These are distin- 

 guished from all other Mollusca by their feet, which, 

 instead of forming a horizontal disk, are compressed 

 into vertical muscular laminae, which they use as fins. 

 They swim horizontally, and can inflate the body 

 with water in a manner which is not yet well under- 

 stood. The only representative of this order is the 

 genus Carinaria, and the species observed here is 

 Carinaria cymhium. 



VI. Gasteropoda PectinibrancMata. — This order 

 comprises almost all the univalve spiral shells, and 

 many that are simply conical; it is consequently 

 the most numerous in species. Cuvier arranges 

 these Mollusca under several families, from the form 

 of their shells, which appear to be in sufficiently con- 

 stant harmony with those of their respective animals. 



The first ^family of Pectinibranchiata, — the Tro- 

 choides — are recognized by their shell, being of a 

 conical form, with the spire more or less elevated, 

 and the base generally flat or concave, rarely convex. 



15—2 



