508 



HISTORICAL GEOLOGY. 



The genus Orthoceras had many later species. But Endoceras, of which 

 there are over twenty described American species, began in the Canadian 

 and ended in the Trenton period. 



683-688. 



AQQ 



-ft , 686^ 



( 'J 



Cephalopods. — Fig. 683, a, Orthoceras junceum ; 684, O. olorus (xi/s) ; 685, Actinoceras Bigsbja ; 686, a, Cyr- 

 toceras subannulatum D'Orb. ; 687, Trocholites undatus ; 688, T. Ammonius. Figs. 683-687, Hall ; 688, D. 



7. Crustaceans. — Trilobites were of varied forms and many new genera ; 

 Asaphus, a Calciferous genus, holds on ; and so also IllcBnus, Ceraurus, and 

 Bathyurus; the latter two have their last species in the Trenton. Fig. 689 

 represents Asaphus platycephalus, from Trenton Falls, N.Y., which is often 



689-693. 690 



Crustaceans. — Fig. 689, Asaphus (I sotelus) platycephalus (x %) ; 690, a, Calymene callicephala ; 691, Lichas 

 Trentonensis ; 692, Trinucleus concentricus ; 693, o, b, Leperditia fabulites (natural size). 689, 691, Hall; 

 690, 692, Meek ; 698, T. E. Oones. 



