THE PERMIAN PERIOD. 655 



ceras (Fig. 306, b and c), and Popanoceras (Fig. 306, d and e) were repre- 

 sentative types, occurring in America, Europe, and Asia. The older 

 types of coiled cephalopods, the goniatites and nautiloids, mingled 

 with these newcomers. Interestingly enough, there was also present 

 the ancient straight form Orthoceras (Fig. 306, /), now reduced to primi- 

 tive simplicity and diminutive size, lingering along in the last stages 

 of its prolonged career, to finally disappear in the Trias. The con- 

 trast (compare Fig. 306, /, with a, b, and d) between the disappear- 

 ing straight type, in its depauperate form, and the robust, youthful 

 ammonites, about to become a ruling dynasty in its stead, is most 

 marked. It was in this, as in not a few other cases, a Permian function 

 to welcome the coming and speed the parting guest. 



The retreatal tracts of the marine life. — As in previous transition 

 epochs when epicontinental waters were largely withdrawn, the marine 

 faunas found special refuge in certain embayments or border tracts 

 which, in connection with the coastal belts, permitted them to re-form 

 themselves, re-generate their species, and prepare for a succeeding 

 invasion of the continental areas. On the American continent, the 

 St. Lawrence embayment had done repeated duty in this line, but 

 there is no specific evidence that it participated notably in the Permo- 

 Triassic transition. The border of the Gulf of Mexico, the Mediter- 

 ranean tract, notably in the region of Sicily and southeast Europe, and 

 the Ganges-Indus tract of southern Asia, seem to have been special 

 areas of refuge and regeneration. There are signs of another in northern 

 Asia, and suggestions of still another in an unknown region from which 

 the Triassic invasion of the Tropitidce came. In these and doubtless 

 other tracts, and on the continental borders generally, the shallow- 

 water marine faunas passed from the Paleozoic to the Mesozoic phases. 

 The restriction, compared with the expansional stage of the Missis- 

 sippian period, was extremely severe, but the faunas emerged with 

 new species born in adversity, ready for conquest when the re-advancing 

 seas should give them an expanding realm. 



The Problems of the Permian. 



Between a marvelous deployment of glaciation, a strangely dispersed deposi- 

 tion of salt and gypsum, an extraordinary development of red beds, a decided 

 change in terrestrial vegetation, a great depletion of marine life, a remarkable 

 shifting of geographic outlines, and a pronounced stage of crustal folding, the 

 events of the Permian period constitute a climacteric combination. Each of 



