LIFE 499 



By the middle of the Triassic period the faunas had begun to 

 intermingle, and to lose their provincial characteristics. The Med- 

 iterranean fauna gained access to the Indian basin and to our 

 western coast, and counter-migrations were of course made possible. 

 At about the same time, the Siberian fauna had access to western 

 United States. 



During the later stages of the period a rich marine fauna 

 flourished in California. Many of its species were identical with 

 those of the Mediterranean and Himalayan regions, or closely allied 

 to them. It is therefore inferred that these provinces were in free 

 communication, so far as marine life was concerned, with the west 

 American coast. 



Prominent types. The most conspicuous feature of the Triassic 

 faunas was the re-ascendancy of the cephalopods in the form of the 

 ammonites (Fig. 421), which had a marvelous development during 

 the period, reaching a thousand species. Their evolution was the 

 more notable because the structural changes were conspicuous, and 

 showed plainly the advance of each stage over the preceding. While 

 early types still persisted, closely coiled, intricately-sutured forms 

 predominated. The first representatives of the cuttlefish type 

 appeared at this time. The deployment of the cephalopods was 

 therefore greater than ever before, though they did not reach their 

 culmination till the next period. Old forms, orthoceratites and 

 goniatites, made their last appearance in thi$ period. The re- 

 markable commingling of old and new types makes this one of the 

 most instructive assemblages in the history of the cephalopods. 



A similar commingling of transitional forms was presented by the 

 gastropods, and the progress of the bivalves was scarcely less real, 

 though they do not show the transition from ancient to modern so 

 conspicuously. Their numbers were large, and most of their genera 

 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 bracMopod types of the late Paleozoic were dis- 

 tinguished by extended hinge-lines, while the narrower beaked or 

 rostrate forms were in a respectable minority. In the Triassic 

 period the latter became predominant, and have remained so ever 

 since. (Compare Figs. 422 and 367.) 



Among echinoderms, leadership passes from the crinoids to the sea- 

 urchins. Starfishes and brittle-stars were present, but not abundant. 



