﻿68 BRITISH FOSSIL CEPHALOPODA. 



This seems to me a very well-marked group of small shells, whose collective 

 characters form as good a generic distinction as any that could be desired. The 

 band along the convex side is universal in all well-preserved species, and is probably 

 essential to the genus. If species without this band and with a well-preserved 

 simple aperture were found, they would form the link to the genus Lituites, which 

 is at present but remotely related. 



Range. — The genus is only known at present in the Upper Silurian rocks of 

 England and Bohemia, unless the species described by Barrande as a Lituites from 

 the Lower Silurian belongs here. 



Genus Oryptoceras, D'Orbigny. 

 1850. Oryptoceras, D'Orbigny, ' Cours de Pal. stratigraphique.' 



History. — This genus was separated from Nautilus, according to D'Orbigny 's 

 general scheme of subdivision, on account of the position of the siphuncle, the 

 present name being ascribed to those in which that organ is external. This 

 character alone being insufficient for distinction, the name has met with little 

 acceptance, but it may be used, as below, to represent a genus typified by the 

 species to which D'Orbigny chiefly referred, i.e. Nautilus dorsalis. 



Description. — The whorls are few and not always in contact, and there is an 

 initial vacuity. The curvature is not continuous, but in the region of the body- 

 chamber, which is also transversely inflated, it diminishes to almost zero, but 

 increases again afterwards. The siphuncle is only known as external, but this may 

 not be essential. The shell is thus, as it were, a Poterioceras, with the curvature of 

 an Ancycloceras. 



Range. — It is not certain if there be any other representative of the genus than 

 the typical Carboniferous species, unless the so-called Cyrtoceras bdellalites of the 

 Devonian belongs to it. 



