THE PENNSYLVANIAN PERIOD. 615 



water; whether also in salt water is uncertain. It is extremely prob- 

 able that the sharks armed with shell-crushing teeth (the cochliodonts 

 and psammodonts) were chiefly marine, and that they continued 

 to be abundant through this period, though probably less dominant 

 than in the extensive shell-bearing seas of the Mississippian, while 

 the sharks armed with cutting teeth were also largely marine, but 

 less dominantly so. There was further progress in adaptation to 

 swift movement and in acquiring shapeliness of outline. 



Changes in the aspects of the mollusks. — By reference to the pelecy- 

 pods represented in Fig. 286, a, b, c, d, e, and /, and by comparison 

 with previous figures, it will be noted that this group had assumed 

 an appreciably more modern aspect. They are scarcely Paleozoic 

 in the sense of being very ancient. The period was doubtless very 

 favorable to their advance, by presenting an expansive but shifting 

 and oscillating habitat, that both invited and forced mutational plas- 

 ticity. The gastropods, on the other hand (Fig. 286, g, h, i, j, k), show 

 less advance over the earlier types. They resemble those of the Gene- 

 vieve and even of the Devonian fauna, though quite unlike those of 

 the Osage. On the whole, they still present a distinctly Paleozoic 

 aspect. 



Association of lingering and advanced cephalopods. — There was a 

 notable juxtaposition of very ancient and of relatively modern cephalo- 

 pod types, the former represented by very straight, plain, small ortho- 

 ceratites (Fig. 286, r) that might well belong to the earliest Paleo- 

 zoic epoch, the latter by closely coiled goniatites (Fig. 286, q) with 

 curved sutures that might well be an initial Mesozoic type. The 

 orthoceratites were about to take their departure, the goniatites 

 were about to evolve into the ammonites, the reigning type of the 

 Mesozoic era. 



The brachiopods. — As usual the brachiopods held an important 

 place, and their general facies was like that of the Upper Mississippian. 

 Productus was the leading form and had many species (Fig. 286, t, u, 

 and v), while Athyris, Chonetes (Fig. 286, z), Spiriferina (Fig. 286, y), 

 and other genera common in the Mississippian persisted in abundance, 

 but in new species. Spirifers also were again common, some of them 

 exhibiting an advanced state of bifurcation of the plications, as in 

 Spirifer cameratus (Fig. 286, dd). Some species of the brachiopods 

 were long-lived, and some ranged not only through northern America 



