CEPHALOPODA. 389 



internal mould and surface ornaments. ' In G. Nereus the tube is more slender, 

 less involute, and the expansions of the test are more frequent and more 

 decidedly plicated, but much less elevated The curvature of the tube in G. 

 Matheri is distinctive, and the tube is more slender, and with a flattened, trans- 

 verse section. This form is a remarkably large and well-defined species, and 

 Las a horizontal range greater than any of the previously described forms, 

 except, perhaps, C amulum. 



Formation and localities. In the limestones of the Upper Helderberg group, 

 Helderberg mountains, N. Y.; near Columbus, 0.; and Kelly's Island, Lake 

 Erie. 



In the historical notices and descriptions of species of Gtroceeas and Cyrto- 

 ceras, no mention has been made of the numerous genera and subgenera which 

 have been constituted on various modifications of the typical forms of these 

 genera. The majority of the species of Ctrtoceras and allied genera have 

 been indicated from the Silurian system. And on the other hand, the greater 

 proportion of the species of Gyroceras have been described from Devonian and 

 Carboniferous strata. From this fact, it would seem probable that there was 

 sufficient grounds for some generic separation, and that these genera held an 

 equivalent relative position in these grand divisions of palasozoic time. 



In the Silurian system the genera Orthoceras and Cyrtoceras form the 

 leading feature in the cephalopodous fauna, while in the succeeding periods 

 the genus Nautilus supersedes Orthoceras, and Gyroceras appears to be a 

 gradual and natural outgrowth from Cyrtoceras. 



