446 



MESOZOIC TIME— REPTILIAN AGE. 



decisive — for referring the upper half of that formation to the Jurassic. The 

 European species of this genus occur in the Lias and Trias. 



The species of Radiates and Mollusks here figured were collected at the Black 



Hills. 



1. Radiates. — Fig. 665, a joint of the stem of Pentacrinus Aster iscus, a 

 Crinoid with a pentagonal column. 



2. Mollusks. — (a.) Oonchifers. — Fig. 666, Monotis curta; fig. 667, Trigonia 

 Conradi; 668, Tancredia Warreniana. (6.) Cephalopods. — Fig. 669, young spe- 



Figs. 665-670. 



Fig. 665, A segment of the column of Pentacrinus asteriscus ; 666, Monotis curta ; 667, Tri- 

 gonia Conradi ; 668, Tancredia Warreniana; 669, Ammonites cordiformis ; 669 a, Side-view 

 of same, a little reduced ; 670, Belemnites densus. 



cimen of Ammonites cordiformis; fig. 669 or, side-view of the same; fig. 670, 

 Belemnites densus, the upper part broken away. 



Among the Arctic fossils of this period, there are at Prince Patrick's Land 

 Ammonites M' CliritocM, a species near A. eoncavits of the Lower Oolite; and at 

 Cook's Inlet, Ammonites Wosnessenski, A. biplexf, Belemnites paxillosus, and 

 Unio Liassinus. A. biplex also is reported to occur in the Chilian Andes, in 

 latitude 34° S., and probably also in Peru near the equator, as well as in Britain 

 and Europe. 



2. FOREIGN. 



I. Rocks : kinds and distribution. 



The strata of the Jurassic period in England (see map, page 354, 

 on which the areas numbered 7, 8, are Jurassic) appear at the surface 

 over a narrow range of country (averaging thirty miles in width) 

 commencing at Lyme-Regis and Portland on the British Channel, 



