﻿432 



Fig. 15. — Nautilus umbilicatus. 



V. Descriptions. 



In treating of the history of the impressed zone it has become 

 essential to describe a number of new genera and also new species. 

 The old names — Gyoceras, Lituites, Nautilus, etc. — convey en- 

 tirely false ideas of affinity and would serve to confuse the student 

 of these fossils, since the new names ignore these groups. Thus it 

 is not at all in contradiction of the nomenclature if I state that the 

 close-coiled Tarphyceras prematurum of the earlier Quebec faunas 

 is the last of its phylum and has no descendants in the Upper 

 Silurian or later; but if I call it Nautilus, and say that Nautilus 



