THE PENNSYLVANIAN PERIOD 657 



fresh water. It seems clear that much the larger number of those 

 found in the American Coal Measures lived in fresh water; whether 

 also in salt water is uncertain. It is probable that the sharks armed 

 with shell-crushing teeth were chiefly marine, and those with cutting 

 teeth largely so. During the period, there was progress among 

 the fishes in adaptation to swift movement, and in shapeliness of 

 form. 



The invertebrates. By comparing the pelecypods represented in 

 Fig. 456, with earlier forms, it will be noted that this group had 

 assumed a more modern aspect. The period was doubtless favor- 

 able to their advance, by presenting an extensive but shifting 

 habitat that both invited and forced adaptive change. The 

 gastropods, on the other hand show less departure from earlier types. 

 They are still distinctly Paleozoic. Ancient and relatively modern 

 types of cephalopods lived together, the former represented by 

 straight, plain, small orthoceratites (z, Fig. 456) that might well 

 belong to the earliest Paleozoic period, and the latter by closely 

 coiled goniatites (zz) with curved sutures that might well be early 

 Mesozoic types. The orthoceratites were about to take their final 

 leave, and the goniatites were about to evolve into ammonites, the 

 dominant type of the Mesozoic era. 



The brachiopods held an important place, and their general 

 facies was like that of the later Mississippian. Some species range 

 not only through northern America and Eurasia, but into the 

 Orient and Australasia. Crinoids were a smaller element of the 

 fauna than might have been expected from their previous and 

 subsequent history. There was a close relation between several 

 American and Russian crinoids, implying intermigration. The 

 cystoids and blastoids were gone. No starfishes have been recog- 

 nized, though they were doubtless present, and sea-urchins were 

 rare. Trilobites, which commanded foremost attention at the 

 opening of the Paleozoic, are now almost at the point of disappear- 

 ance. The last representative of the group had the chaste beauty 

 of its early ancestors. Bryozoans were not uncommon, but the 

 peculiar devices for support illustrated in Archimedes and Lyropora 

 of the preceding period were abandoned. Protozoans were repre- 



