FOSSIL VERTEBRATES — FISHES 4*5 



peripheral portion of the ring is seen in the more primitive 

 forms, Microdon and Pycnodus. In the first of these the bases 

 of the arches are expanded and terminate in rounded and 

 flattened processes that cover the sides of the notochord to a 

 considerable extent, but do not meet in the middle line. In the 

 second form the bases of the arches are more expanded than in 

 the first and the edges of the expanded portions are serrated so 

 that they interlock both with the ones immediately before and 

 behind them and with the one on the opposite side of the noto- 

 chord. 



There have been mentioned here, of course, only the forms 

 that show to some extent the modifications and the lines of 

 development along which the Ganoids traveled. The waters of 

 the Mesozoic lakes and oceans were swarming with members of 

 the group that presented almost as many varieties of structure 

 and form as do the modern bony forms. Because of the strong, 

 interlocking, enameled scales the whole body of the fish is com- 

 monly preserved, but the internal skeleton is much less com- 

 monly available, so that the most of the forms are known from 

 characters of the scales and the position of the fins, both of 

 which characters are, to a certain extent, unsatisfactory and 

 unreliable. 



The Teleocephali, the group generally known as the Teleosts, 

 seem to have appeared at about the beginning of the Mesozoic 

 time though it did not reach a great degree of development 

 until near the close of that period and during the Tertiary time. 

 The members of this group differ from the other forms of fishes 

 in the complete calcification of the bones of the body and the 

 nearly complete loss of the notochord by the development of 

 the solid vertebrae which divides it up into intervertebral seg- 

 ments. The scales are horny and rounded, loosely attached to 

 the skin and overlapping in the style of shingles. The fins are 

 formed almost entirely by the dermal fin rays, the basals, and 

 the radials being greatly reduced. Dean says, p. 167 : " Fins 

 are dermal structures, their ancient basal supports hardly to be 

 distinguished ; the primitive tail structure is so masked by 



