^9 



GASTEROPODA. 



Fig. 405. — Holopca 

 Ouclphensis. (Billings.) 

 Middle Silurian. 



Silurian and Devonian, and includes thin, conical, often 

 smooth shells, with an expanded body -whorl (fig. 405). 

 The mouth of the shell is entire, and the 

 peristome seems to be sometimes complete, 

 as in the Scalaridcc. The Devonian Isonema 

 has angular whorls and a rhombic mouth, 

 but in other respects resembles Holopea. 

 In the Silurian and Devonian Platyostoma, 

 again, the body -whorl is immensely ex- 

 panded. Lastly, the genus Cydonema, 

 ranging from the base of the Silurian to 

 the top of the Devonian, has a conical shell, 

 which is characterised by the possession of fine spiral stri«, 

 often with transverse stride as well. 



In the genus Solarium, comprising the " Staircase Shells," 

 the shell (fig. 406) is much depressed; there is a large and 



deep umbilicus, running from the 

 base to the apex of the shell ; and 

 the aperture is rhombic. The edge 

 of the umbilicus is, typically, crenu- 

 lated ; and the shell is not pearly 

 within. The genus appears in the 

 Secondary period, is represented by 

 undoubted species in the Cretaceous, 

 is not uncommon in the Tertiaries, 

 and survives at the present day. 

 Bifrontia, Eocene and Eecent, has 

 the body-whorl free. Fhorus (fig. 

 407) comprises trochoid shells, with 

 a concave base and flattened whorls. 

 Very usually foreign bodies, such as 

 shells or small pieces of stone, are attached to the surface 

 and margins of the shell. There is considerable uncertainty 

 as to the geological range of the genus, species having been 

 described from deposits as old as the Devonian ; but the 

 first undoubted forms occur in tlie Jurassic, and there are 

 various Tertiary and living types. We may also place here 

 the genus Cirrus (fig. 408), in which the shell is discoidal 

 and there is a large umbilicus ; while the upper surface 



'.L 



Fig. 406. — Solurhmi ornatum. 

 Gault (Upper Cretaceous). 



