THE PERMIAN PERIOD. 



653 



land, and restriction in sea, when normally the relations are reciprocal. 

 In North America, the restricted marine area lay in the tracts just 

 previously occupied by the expanded Carboniferous seas, and inherited 

 its fauna from them. At first nearly all the species were the same 

 as those of the preceding period, and hence a grave difficulty has always 

 been experienced in drawing a dividing line between them. Prosser, 

 who has carefully studied the Permian of the great plains, has recently 

 listed the known species, and found only about seventy. This sug- 

 gests a decadent fauna. Of these, about one half are pelecypods, 



Fig. 305. — A Permian fish, Platysomus gibbosus, restoration by Traquair, about one- 

 fourth natural size. Germany and England. (Woodward.) 



which is doubtless a response to the increasing turbidity that attended 

 the shallowing of the sea. Representative forms are shown in 

 Fig. 306, g-j. Gastropods (Fig. 306, k), brachiopods, and cephalo- 

 pods (Fig. 306, a-f) make up the larger part of the rest. Generally, 

 the productids were the most characteristic forms, and the long-lived, 

 cosmopolitan Productus semireticulatus frequented the seas in all 

 quarters of the globe. It was the last flourishing stage of the pro- 

 ductids, as well as of the orthids, the spirifers, and the athyrids, though 

 the last two eked out a lingering existence till the Jurassic period. 

 The advent of the ammonites. — The increasing complexity of the 

 sutures of the coiled cephalopods has been noted in previous periods. 

 In the goniatites of the Middle Devonian, the sutures had become 



