50 THECOSOMATA. 



class of beings is at present not very well-defined. On 

 account of the arms and suckers in certain genera, and 

 from a belief that the intermediate lobe, which in Spinalis 

 and lieterofusus bears the operculum, represents the foot, 

 we have preferred to follow Cuvier and Rang in placing 

 them between the Cephalopods and Gasteropods; Lamarck 

 conceived they should be between the Gasteropods and 

 Bivalves; M. Elainville maintained they were a tribe of 

 Gasteropods allied to the BuUidts, and this opinion has 

 recently been supported by M. Souleyet. 



Order THECOSOMATA. 



Head indistinct, with two wings on the sides of the 

 mouth. Tooth of lingual membrane hooked, with a strong 

 hooked tooth on each side. Gills internal. Body inclosed 

 in a shell. 



This order may be divided into two sub-orders, including 

 those with the body and shell straight, or globular, and 

 without an operculum ; and those with the animal and shell 

 spiral, and with a spiral operculum. The families Cavo- 

 linidfB and TripteridtK have calcareous shells, while in 

 CymbuliidfB the shell is cartilaginous ; the second sub-order 

 contains the single family Limacinidie. 



Fam. OAVOLINID^. 



Animal with two united fins without any posterior foot- 

 like appendage between them. Abdomen voluminous. Gills 

 in pairs. Internal superior organs of generation on the 

 right side. 



Shell calcareous, symmetrical, elongate, or globular. 



In the genera Cavolina and Diacria the shell has a 



