285 



described nearly two hundred and fifty species from rocks of 

 this age in Bohemia. 



Fig. 255. Pkragmoceras(Campulites)ventricosns. Upper Silurian. The right- 

 hand figure shows the form of the aperture. 



In the genus Phragmoceras 

 (fig. 255) the shell is curved, 



and its aperture is contracted 

 in the most extraordinary 

 manner in the middle, so as 

 to assume somewhat of the 

 shape of a keyhole. The 

 siphuncle in the majority of 

 cases is placed upon the con- 

 cave side of the shell. In 

 other cases, the ventral side 

 of the Mollusc corresponds 

 with the convex side of the 

 shell. Th e species of Phrag- 

 moceras are Silurian and De- 

 vonian, mainly the former ; 

 and M. Barrande enumerates 

 thirty-eight species as occur- 

 ring in the Silurian basin of 

 Bohemia. 



In the genus Gomphoceras, 

 the shell is spindle-shaped, or 

 globular, tapering towards its 

 apex. The aperture is con- 

 tracted in the middle, like that of Phragmoceras, and the 

 siphuncle is generally sub-central. In most cases the ventral 



Fig. 256. Asc 

 lings), showing t 

 Lower Silurian. 



-as Canadensis (Bil- 

 he form of the septa. 



