PTEROPODA. 803 



The members of the Firolidce are either shell-less, or possess a 

 small hyaline shell, placed on the back, protecting the gills. The 

 only member of this family which is known to be certainly repre- 

 sented in the fossil state is the existing genus Carinaria (fig. 717), 

 a single species of which has been found in deposits of Miocene 

 age. 



In the AtlantidcB the animal is furnished with a well-developed 

 but fragile shell, completely enclosing the body. In Atlanta itself 

 the shell is rolled into a flat spiral, and the body-whorl is keeled, 

 and exhibits a dorsal slit at the aperture, thus resembling the shell 

 of the Belleropho?itid<z. A single species of Atlanta has been found 

 in the Tertiary deposits of San Domingo. Owing to the close re- 

 semblance of the shell of Bellerophon and its allies in external form 

 to that of Atlanta, the former have been commonly regarded as 

 Heteropods. The comparative thickness and solidity of the shell 

 of the Belleropho7itid&, however, and the common association of the 

 forms of this family with fossils of an unquestionable shallow-water 

 type, forbid the reference of these singular Palaeozoic Gastropods 

 to the present order. 



Order IV. Pteropoda. 



The Pteropods or "Winged Snails" are pelagic Molluscs, found 

 swimming near the surface in the open sea, the living forms being 

 all of small size. They have no distinctly differentiated head, and 

 the mouth is placed anteriorly in the centre of the fore-part of the 

 foot, which is rudimentary. The lateral parts of the foot ("epi- 

 podia") are, however, developed into a pair of wing-like fins (fig. 

 718), by means of which the animal swims actively. The posterior 

 part of the foot (" metapodium ") is 

 rudimentary, but in some cases may 

 develop an operculum. In some 

 groups (Gymnosoniata) the mantle is -^ 

 rudimentary, and the adult is not pro- 

 vided with a shell. In other groups 

 (Thecosomata) the mantle is well de- 

 veloped, and secretes a shell. In both 

 sections of the order the embryo is 



i Fig. 718.— Cavohnia {Hyala-a) trt- 



IUmished With a Shell, but the larval dentata, showing the shell, and the 



i n • 1 . • , 1 /-< , lateral fins attached to the sides of the 



shell is soon lost in the Gymnosomata ; head ^ ff y 



whereas in the great majority of the 



Thecosoinata, the adult develops a secondary and permanent shell, 



of which the embryonic shell usually forms the initial portion. The 



shell of the Thecosomata generally is calcareous in composition, and 



in all the living forms is very delicate in texture. The so-called 



