676 GEOLOGY 



drawing a dividing line between them. The known species of the 

 Permian of the Great Plains are only about 70, and of these about 

 one-half are pelecypods. Among the brachiopods, the product ids 

 were the most characteristic forms. This system records the 1 

 flourishing stage of the productids, the orthids, the spirifers, and 

 the arthyrids, types which had a long history. 



The increasing complexity of the sutures of the coiled cephalopods 

 has been noted in previous chapters. In the later part of the Per- 

 mian, the complexity became still greater (Fig. 463), foreshadowing 

 the intricacy of the Mesozoic ammonites; but older types (gonia- 

 tites and nautiloids) mingled with the new. Interestingly enough, 

 there was also the ancient straight form (Orthoceras, /, Fig. 463), 

 in the last stage of its prolonged career. The contrast (compare 

 a, Fig. 463) between the disappearing straight type, in its depau- 

 perate form, and the robust youthful ammonites, about to become 

 a ruling dynasty, is marked. 



The retreatal tracts of the marine life. As in previous transi- 

 tion epochs when epicontinental waters were largely withdrawn, 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, regenerate their species, and prepare 

 for a succeeding invasion of the continental areas. On the Amer- 

 ican continent, the St. Lawrence embayment had done repeated 

 duty in this line ; but there is no specific evidence that it participa t < M 1 

 notably in the Permo-Triassic transition. The border of the (lulf 

 of Mexico, the Mediterranean 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. Here 

 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 Mississippian 

 period, was great; but the faunas emerged with new species born 

 in adversity, ready for conquest when the re-advancing seas should 

 give them an expanding realm. Unfortunately, the sediments in 

 which this transition of faunas should be recorded, are, lor the most 

 part, buried and inaccessible. 



