THE TRIASSIC PERIOD 701 



While corals were rare in most places, they were rather abundant 

 in favored localities. Some of them resembled the Paleozoic forms 

 in being simple and cup-shaped, but the compound species took on 

 the modern (hexacoralla) form, and the compound Paleozoic (tetra- 

 coralla) type disappeared. These later compound corals do not 

 seem to have descended from the compound Paleozoic forms, but 

 from some simple type. 



While the general aspect of the Triassic marine faunas was 

 revolutionary, it is important to note, in view of beliefs once cur- 

 rent, that it was transitional, and not an abrupt substitution of a 

 new fauna for an old one. Paleozoic types lived side by side with 

 later forms, though usually represented by new genera. This 

 overlapping and commingling of old and new clearly indicates 

 the gradation of the earlier into the later. The transition was 

 extraordinary in the apparent rapidity of its progress, and in the 

 extent to which it affected all classes. The fact that most of 

 the new types were already present in the earliest Triassic, indi- 

 cates that the transition was chiefly in the Permian. The funda- 

 mental cause was, with little doubt, the readjustment of the earth's 

 surface to internal stresses, and the physiographic and climatic 

 changes consequent upon this readjustment. 



Marine reptiles seem to have thriven in the western part of 

 our country, especially in the middle and later Trias. The numerous 

 ichthyosaurs found in the later Triassic beds, and their peculiarities, 

 suggest that this may have been a center of dispersion of these 

 reptiles. With the ichthyosaurs were other reptiles (thalattosaurs) 

 unknown elsewhere. 1 



1 Merriam, The Thalattosauria, Cal. Acad. of Sci.; Triassic Ichthyosauria: 

 Memoirs of the Univ. of Cal., 1908. 



