HETEROPODA AND PULMONIFERA. 



39 



Atlantidce. The former of these is represented by a single 

 species only, from the Miocene Tertiary. The latter had a 

 great development in Palseozoic seas, and is represented in 

 the formations of this period by several remarkable genera. 



Eam. 1. FiROLiD.^L — Body large, never completely pro- 

 tected by a shell, often shell-less. Sometimes a small deli- 

 cate hyaline shell, placed on the back, protecting the gills. 

 The only genus of this family which is kno^vn to be certainly 

 represented in a fossil state is Carinaria (fig. 427), a single 

 species of which has been found in deposits of Tertiary age 

 (Miocene). 



Fam. 2. Atlantidj^. — Animal furnished with a well-de- 

 veloped shell, into which it can retire. Shell symmetrical, 

 discoidal, destitute of septa, often provided with an opercu- 

 lum. This family is represented by the genera Bclhro'plion, 

 Madurca, Cyrtolitcs, jEccvMomj^hcdus, &c., most of which are 

 exclusively PahTcozoic, whilst the others are mainly so. 



In the genus Bellerojjhon (fig. 428) the shell is symmetri- 



Fig. 428. — Bdleroplwn Argo (Billings), a, Front view ; b. Side view. Lower Silurian. 



cal, convoluted, the coils of the shell usually lying in one 

 plane. The whorls are few, smooth or sculptured, and there 

 is a dorsal keel along the convex margin of the shell. The 

 aperture is often more or less expanded, and is in most in- 

 stances emarginate or deeply notched on the dorsal side. 

 The genus ranges from the Lower Silurian to the Carbon- 

 iferous. The Bellero2Dhina of the Gault (Upper Cretaceous) 

 is doubtfully allied to BcUcroplion, and may belong to the 

 PtcroiJoda. Bucania, of the Silurian, includes forms not 

 generically separable from Bellero2olion, but distinguished by 

 the fact that all the volutions are visible and increase in 



