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CHAPTER LII. 



CLASS REPTILIA. 



General Structure. 



With the Reptiles we enter upon the consideration of the first of 

 two classes which, from the possession of many common characters, 

 have been brigaded together by Professor Huxley under the name 

 of Sauropsida. The name Monocondylia had, however, been pre- 

 viously proposed by Hseckel for these two classes, and some writers 

 consequently prefer to use this term. These two classes are the 

 Reptilia and the Aves, or Birds ; and although recent research has 

 shown the existence of a close affinity between the more generalised 

 Reptilia and the Amphibia, and thus with the Mammalia, yet it has 

 in no wise tended to interfere with this association. It should, how- 

 ever, be observed that since it is probable the Reptiles have taken 

 origin from forms more or less nearly allied to some of the earlier Am- 

 phibia, with which we are at present acquainted, it is obvious that 

 there must once have existed animals in which the characteristic fea- 

 tures of the true Amphibia and the Reptilia were more or less blended, 

 and that the practicability of drawing a distinction between the two 

 classes is thus (as in other cases) more or less due to the imperfec- 

 tion of our knowledge. With this proviso, and bearing in mind that 

 some of the more generalised forms with which we are even now 

 acquainted may not conform in every detail with the undermentioned 

 characters, the Reptilia as a whole, together with the Birds, may be 

 distinguished from the preceding classes on the one hand, and from 

 the Mammalia on the other, by the following features. 



Epidermal structures in the form of scales or feathers are gener- 

 ally present, but there are never hairs. The vertebrae, which are 

 ossified, usually have no epiphyses. 1 The basioccipital, with one 



1 These are present in some of the wSauropterygia among Reptiles, and in 

 Parrots among Birds. 



