THE TRIASSIC PERIOD 699 



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. The Upper Trias of British Columbia, on the other 

 hand, contains a different fauna, including a type that belongs to 

 the Siberian group. The British Columbian fauna is perhaps to be 

 regarded as the descendant of the Idaho fauna of the early Trias, 

 with additions from Siberian sources, while the California fauna is 

 perhaps a derivative from the Mediterranean and Himalayan prov- 

 inces by some different route. Present knowledge, however, is not 

 sufficient to show the precise nature of the migrations between 

 Europe, Asia, and America during Triassic times. 



The most conspicuous feature of the Triassic faunas was the re- 

 ascendancy of the cephalopods in the form of the ammonites, which 

 had a marvellous development during the period, reaching a thou- 

 sand 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, the closely coiled, intricately-sutured forms predominated. 

 The first known cephalopods of the cuttlefish type appeared at this 

 time. The deployment of the cephalopods was therefore more 

 varied and comprehensive than ever before, though they did not 

 reach their culmination till the next period. Old forms, ortho- 

 ceratites and goniatites, made their last appearance in this period. 

 The remarkable commingling of old and new types makes this one 

 of the most instructive assemblages in the history of the cepha- 

 lopods. 



A similar commingling of transitional forms was presented by 



f+ ^e gastropods. The progress of the bivalves was scarcely less real, 

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

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



