NAUTILID^. 



71 



has about the same vertical range as Orthoceras, ranging from 

 the Silurian through all tlie Palaeozoic formations, and dis- 

 appearing in the Trias. The genus is 

 characteristically Silurian, and M. Bar- 

 rande has described nearly two hundred 

 and fifty species from rocks of this age 

 in Bohemia. 



In the genus Phragmoceras (fig. 458) 

 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 concave side of the 

 shell. In other cases, the ventral side 

 of the Mollusc corresponds with the con- 

 vex side of the shell. The species of 

 Phragmoceras are Silurian and Devonian, mainly the former ; 

 and M. Barrande enumerates thirty-eight species as occurring 

 in the Silurian basin of Bohemia. 



Fig. 457. — Cyrtoceras iso- 

 donis. (Billings.) Lower 

 Silurian. 



Fig. 45S. — Phragmoceras (Campjdites) ventricosics. Upper Silurian. Tlie right-hand 

 figure shows the form of the aperture. 



In the genus Goviplioceras the shell (fig. 460, A and b) is 

 spindle-shaped, or globular, tapering towards its apex. The 

 aperture is contracted in the middle, like that of Phrag- 

 moceras, and the siphuncle is generally sub-central. In most 



