522 



HISTORICAL GEOLOGY 



local abundance seemed, for a time, to give promise of regaining the 

 former importance of the race. Sea urchins, which for the millions 

 of years of the Paleozoic had remained in a subordinate position, were 

 replaced by forms of modern structure and occupied, to some degree, 

 the place vacated by the crinoids, cystoids, and blastoids. 



The brachiopods of the Paleozoic were, as far as external appear- 

 ance is concerned, of two classes ; those with long hinge lines, giving 

 them a square-shouldered look, and those with short hinge lines and 

 sloping shoulders. Of these, the square-shouldered and most charac- 

 teristic type soon disappeared, and only a comparatively few genera 

 of the sloping-shouldered type survived. The brachiopods, as was 

 true of so many other classes, after attaining considerable importance 

 gave way to other classes of animals. In this case, as they decreased 

 in abundance, the pelecypods and gastropods increased. The com- 

 paratively simple-sutured cephalopods of the Paleozoic, such as the 

 Orthoceras and the angled goniatite (p. 459), were quite suddenly re- 

 placed by an abundance of cephalopods with complicated sutures. 

 The Orthoceras, which lived throughout the whole of the Paleozoic, 

 had a few survivors in the Triassic, but these soon became extinct. 



The fishes of the Triassic resemble those of the Permian in most 

 particulars, but many of the Permian genera are wanting. The Age 

 of Amphibians passed with the Permian ; and although the Stego- 

 cephalia (p. 485) lived on into the Triassic, they disappeared before 

 its close ; and the insignificant frogs and salamanders of the present 

 are the sole representatives of that once varied and conspicuous 

 race. 



The cause of the revolution in life at the close of the Paleozoic, as 

 has been seen, must be found in the very different physical conditions 

 which were present at this time, since not one or two but many orders 

 of animals and plants either became extinct or were profoundly 

 affected. The formation of great mountain ranges, the withdrawal 

 of epicontinental seas in America, Europe, and elsewhere, must 

 have produced a climate markedly different from that of the Carbon- 

 iferous, since the ocean currents, with their great stores of heat, would 

 be forced to take courses different from those which they formerly 

 held. Moreover, the circulation of the air would be affected by the 

 high mountain ranges. Besides these more evident causes, it is 

 possible that a radical change in climate resulted from the withdrawal 

 of carbon dioxide during the Carboniferous, and that this, combined 

 with the above-mentioned and other physical changes, caused the 



