SUB-KINGDOM I.— VERTEBRATA. 



CLASS III.—REPTILIA. 



By R. Lydekkek, B.A., F.R.S., V.RG.S., &c. 



From both mammals and birds, reptiles are broadly distinguished by the 

 coldness of their blood, which has a temperature but little, if at all, raised 

 above that of the surrounding air. Probably to the popular mind this is the 

 most obvious distinction of reptiles ; and, in the ordinary acceptation of the 

 term, it does serve to differentiate the group from all other vertebrates 

 except fishes — with which these creatures are not likely to bo confounded. 

 But it must be remembered that the popular and scientific conceptions of 

 what constitutes a reptile are totally distinct. Under that title the ordinary 

 observer includes frogs, salamanders, and newts ; and, in fact, all terrestrial, 

 cold-blooded vertebrates. From certain structural peculiarities the natural- 

 ist, on the other hand, separates the creatures just named from reptiles, and in- 

 cludes under that title only crocodiles, tortoises, turtles, lizards and snakes, with 

 a number of extinct forms with which we are not concerned in this volume. 



Although some reptiles, such as lizards, agree very closely with the egg- 

 laying mammals (Monotremata) in the structure of the bones forming the 

 shoulder-girdle, while this resemblance is still more markedly displayed by 

 certain extinct members of the reptilian class, yet the relationships of 

 modern reptiles are decidedly closer with birds. Both groups, for instance, 

 agree in that the young are not nourished with milk ; while as regards 

 structure there are several points in which they are identical, and at the 

 same time differ from mammals. Notably, each branch of the lower jaw in 

 birds and reptiles is composed of several distinct bones ; and the whole jaw 

 is jointed on each side to the skull by means of a separate ossification known 

 as the quadrate -bone. In both groups the ankle-joint is situated between 

 the two horizontal rows of b(mes constituting the tarsus ; although in birds, 

 as well as in some extinct reptiles, these bones are welded respectively with 

 the long bones of the leg and of the metatarsus, A further distincticjn 

 is the articulation of the skull to the first vertebra by means of a single 

 knob-like condyle, instead of by the two condyles characterising mammals. 



Such are some of the features common to birds and reptiles, as distinct 

 from mammals. From birds, reptiles are broadly distinguished by the ab- 

 sence of feathers, the protective structures developed in the outer layer of 

 the skin taking the form either of overlapping horny scales, or of large 

 shield-like plates united by their edges. As already said, reptiles are further 

 distinguished by their cold blood ; but this is obviously a feature of much 

 loss importance, for even among mammals the Monotremata have the tem- 

 perature of the blood perceptibly lower than is the case with the higher 

 members of the class. An important difference is, however, found in the 

 great blood-vessel, or aorta, by which the blood passes from the heart to the 

 body ; since, whereas in birds this is single and bends over the left branch of 



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