700 



GEOLOGY 



modern, some being identical with those now living. With the 

 modern types there were about half as many that still bore a Paleo- 

 zoic aspect. 



The dominant brachiopod types of the late Paleozoic were dis- 

 tinguished by extended hinge lines (spirifers, orthids, etc.), while 

 the narrower beaked or rostrate forms were in a respectable mi- 

 nority. In the Triassic period, the rostrate forms (Rhynchonella, 



Fig. 474. A Group of Marine Triassic Fossils, a, b, and c, cephalo- 

 poda: a, Ceratites whitneyi Gabb; b, Orlhoceras blakei Gabb; c, M <>!:<>- 

 ceras. d-g, pelecypods: d, Corbula blakei Gabb; e, Myophoria alia (Jabb; 

 /, Myacites humboldtensis Gabb; g, Pecten deformis Gabb; hand i, bnn-hi- 

 opods: h, Rhynchonella cequiplicata Gabb; i, Terebratula humboldtensis 

 Gabb. 



Terebratula, and allied genera) became predominant, and have 

 remained so ever since. (Compare Figs. 474 and 419.) 



Although echinoderms are not abundant in the Triassic fauna, 

 the period marks the transfer of leadership from the crinoids to the 

 sea-urchins, and a structural change in the latter. Beginning with 

 the Triassic, the echinoids had twenty rows of plates in belts of two 

 rows each, whereas the Paleozoic forms had more. At first they 

 retained the previous pentamerus symmetry, but later this gave 

 place to a bilateral symmetry. Starfishes and brittle-stars were 

 present, but not abundant. 



