4 68 PENNSYLVANIAN PERIOD 



Marine Life 



Two phases of sea life are worthy of note, (i) that which occupied 

 the shallow water, which, in the form of estuaries, lagoons, and 

 shoals, crept in and out on the borders of the continent as the rela- 

 tions of land and sea oscillated, and (2) the life of the more open seas. 

 No doubt this distinction had existed always, but it had not before 

 reached equal importance. In the coal regions of this period, a large 

 part of the fossils are of shallow water types. In shallow water, 

 where sandy and muddy flats prevailed, pelecypods and gastropods, 

 together with certain fishes, predominated, while in the more open 

 seas the brachiopods, cephalopods, and clear-water types were 

 more plentiful. During the period, there was progress among the 

 fishes in adaptation to swift movement, and in shapeliness of form. 



It is difficult to tell which of them were marine, which fresh water, 

 and which common to salt and fresh water. It is clear that much the 

 larger number of those in the American Coal Measures lived in 

 fresh water; whether also in salt water is uncertain. 



Fig. 404 shows a group of Pennsylvanian marine fossils. It 

 may be noted that ancient and relatively modern types of cephalo- 

 pods lived together, the former represented by straight, plain, small 

 orthoceratites (z, Fig. 404), and the latter by closely coiled goniatites 

 (zz), with curved sutures. The former were about to take their 

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

 the dominant type of the Mesozoic era. Brachiopods were abun- 

 dant, 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. A close relation between sev- 

 eral American and Russian crinoids implies inter migration. Cyst- 

 oids and blastoids were gone, and other forms of echinoderms 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- 

 sented widely by a little foraminiferal shell (Fusulina secalicus, b, 

 Fig. 404), which had about the size and form of a grain of wheat. 

 Its abundance gives character to the Fusulina limestone which occurs 

 in America, Europe, and Asia. Corals were rare, as might be ex- 

 pected under the conditions of the time. 



