70 PLACOID FISHES OF CARBONIFEROUS SERIES. 



There is some reason to think, from the character of the other 

 fossils with which its remains are associated, that this Fish was 

 an inhabitant of fresh water, like the comparatively diminutive 

 Lepidosteus of the present day ( 572). The Sauroid fishes pre- 

 dominate in an increasing degree among the genera of the 

 Ganoid order, as we rise through the newer strata ; and at 

 last they become the only representatives of that order. It is 

 between the Oolitic and Chalk periods, that we find the most 

 remarkable change in the proportion which the Fishes of this 

 group bear to those of other orders ; for, whilst the Oolite 

 includes the remains of numerous fishes of the Ganoid order, 

 many of them Sauroid fishes of great size and strength, we find 

 but very few in the Chalk and later formations, and even these 

 are of diminished size and ferocity; so that, by this gradual 

 change, the order has now become nearly extinct, as already 

 mentioned. 



589. It is in the strata of the Carboniferous order, or the 

 Mountain Limestone and the overlying beds of the Coal series, 

 that we encounter the first appearance, in any considerable 

 proportion, of the Fishes of the Second order, or Placoidians. 

 These do not depart so widely from the forms with which we 

 are familiar at the present day, as did the earlier Ganoidian 

 Fishes ; but it is interesting to remark, that the greater number 

 of the early Placoidians did not bear a resemblance to the Sharks 

 and Rays which are most abundant at the present time, but to 

 some that are now regarded as aberrant forms, separated from 

 the rest by peculiarities of conformation. This is the case, for 

 example, with the Cestracion Philippi., or Port-Jackson Shark ; 

 which has the margins and inner surface of the jaws covered 

 with flat pavement-like teeth, disposed in an oblique row ; 

 whilst the front of the moutli is armed with sharp, angular, and 

 pointed teeth, more resembling those of the ordinary Sharks. 

 The latter are evidently adapted for seizing and retaining the 

 food ; the former for crushing and bruising it. These teetli are 

 rarely found connected together in a fossil state. Now, of the 

 fossil Fishes presenting these peculiarities, and referred on that 

 account to the family Cestracionts, remains are found even in the 



