GASTEROPODA. 



81 



are slender and conical ; Pterotheea has a wing-like expansion ; 

 and Conularia (fig. 26, 10) is a fonr-sided sheath, with the 

 apex partitioned off, as in the recent Cuvieria. If really 

 pteropodous, these shells are the giants of the order. 



Sub-Class.— GASTEKOPODA.* 



In the encephalous Mollusca grouped together under the 

 above name, the muscular disc for creeping is developed from 

 more or less of the ventral surface of the body. 



Aberrant Order. 



In the Nucleobranchiata, here exemplified by the oceanic 

 Atlanta (fig. 24), the ventral foot is as little developed as in 

 Pneumodermon and some other Pterojwda. 



Fig. 24. 

 Atlanta Keraudrenii (magn.) 



There are two existing families of nucleobranchiate Mol- 

 lusks ; the Firolidce, with large and unprotected bodies ; and 



* Gr., gaster, belly ; pous, foot. 



