500 



THE TRIASSIC PERIOD 



Corals were rare in most places, but abundant in favored lo- 

 calities. Some of them resembled Paleozoic forms in being simple 

 and cup-shaped, but compound species took on the modern (hexa- 

 coralla) form, and the compound Paleozoic (tetracoralla) type 



Fig. 422. GROUP OF MARINE TRIASSIC FOSSILS, a, b, and c, cephalopods: a, 

 Ccralites whitncyi Gabb; b, Orthoceras blakei Gabb; c, Mcckoceras. d-g, pelecypods: 

 d, Corbnla blakei Gabb; e, Myopharia alia Gabb;/, Myacites humboldtcnsis Gabb; 

 g, Pecten hnmboldtensis Gabb; h and i, brachiopods: h, Rhynchonella cequiplicata 

 Gabb; i, Terebratula dcformis Gabb. 



disappeared. These later compound corals do not seem to have 

 descended from the compound Paleozoic forms, but from some simple 

 type. 



Though the general aspect of the Triassic marine faunas was 

 revolutionary, it was yet transitional, and not a new fauna substi- 

 tuted for an old one. Paleozoic types lived side by side with later 

 forms, though in most cases represented by new genera. This over- 

 lapping and commingling of old and new indicates clearly the grada- 

 tion 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 lived 

 at the beginning of the Triassic indicates that the transition was 

 chiefly in the Permian. The fundamental 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. 



