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 knowledge of those of the 

 Placoidei. 



Present reptiles form a mere fragmentary remnant of the great 

 and varied class of cold-blooded air-breathing vei'tebrates which 

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

 groups of the class, indicated by osteal and dental chai-acters, have 

 perished ; and it is only by petrified fffices 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 Cuvier,' 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- 

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

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



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

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

 action reciproque. Aueune de ces parties ne pent changer sans que les autres ehangent 

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



autres.' Discours sur hs Eevolutions de la Sur/aie du Glube. 4to. 1826, p. 4-7. In 



tliis definition Cuvier apprehended, exclusiTely, the operanee of the differencing and 

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



