PREFACE. xxvii 



tomy,' I would remark that the existing kinds of vertebrates 

 constitute part only, perhaps but a small proportion, of those 

 which have lived. Two large primary groups of fishes have 

 almost wholly passed away ; but the Polypterus, Lepidosteus, and 

 sturgeon yield the anatomist some insight into the structural 

 modifications of the Ganoidei of Agassiz ; whilst the shark, the 

 skate, and the cestracion give a fuller knoAvledge of those of the 

 Placoidei. 



Present reptiles form a mere fragmentary remnant of the great 

 and varied class of cold-blooded air-breathing vertebrates which 

 prevailed in the mesozoic age. More than half of the ordinal 

 groups of the class, indicated by osteal and dental characters, have 

 perished ; and it is only by petrified feces or casts of the intestinal 

 canal, by casts of the brain-case, or by correlative deductions from 

 characters of the petrifiable remains, that we are enabled to gain 

 any glimpse of the anatomical conditions of the soft parts of such 

 extinct species : by such light some of the perishable structures of 

 these animals are indicated in the text. 



As vertebrates rise in the scale and the adaptive principle pre- 

 dominates, the law of correlation, as enunciated by Cuviee, 1 be- 

 comes more operative. In the jaws of the lion, e.g., there are 

 large laniaries or canines, formed to pierce, lacerate, and retain its 

 prey. There are also compressed trenchant flesh-cutting teeth, which 

 play upon each other like scissor-blades in the movement of the 

 lower upon the upper jaw. The lower jaw is short and strong ; , 

 it articulates to the skull by a transversely extended convexity or 

 condyle, received into a corresponding concavity, forming a close- 

 fitting joint, which gives a firm attachment to the jaw, but almost 

 restricts it to the movements of opening and closing the mouth. 



1 ' Tout etre organise forme un ensemble, un systeme unique et elos, dont les parties 

 se correspondent mutuellement, et concourent a la meme action definitive par une re- 

 action reciproque. Ancune de ces parties ne peut changer sans que les autres cliangent 

 aussi ; et par consequent chaeune d'elles, prise separement, indique et donne toutes les 

 autres.' — Discours sur les Revolutions de la Surface du Globe. 4to. 1826, p. 47. In 

 this definition Cuvier apprehended, exclusively, the operance of the differencing and 

 adapting pole, and the law becomes limited in its application accordingly. 



