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 
has 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 Gyroceras 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 Cyrtoceras 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 palaeozoic 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. 
