HIGH SCHOOL ZOOLOGY. 99 



CHAPTER IV. 



The Reptilia. 



1. Our study of the Catfish and Menobranch has taught us 

 that in addition to the aquatic habits which these creatures 

 shai-e, there are certain anatomical features in which they are 

 alike. The Classes Batrachia and Pisces are not separated from 

 each other by any gulf such as meets us when we advance to 

 the study of the Reptilia. It was possible to point out the 

 existence of living forms intermediate in many ways between 

 the Batrachians and Fishes, but we have no such living forms to 

 bridge over the gap between the Batrachia and the Reptiles. 

 Nor do we know any fossil remains which do so ; on the other 

 hand, the Reptiles and Birds, at first sight so entirely unlike 

 each other in structure as well as habits, are, nevertheless, 

 closely allied by fossil forms wliich present all the important 

 stages of transition between the two groups. Zoologists give 

 expression to these relationships by uniting the Classes Pisces 

 and Batrachia into a group Ichthyopsida, and the Classes Rep- 

 tilia and Aves into a group Sauropsida- The proof of the 

 reptilian affinities of the Birds we shall postpone until we have 

 studied the structure of some of our common Re])tiles. In the 

 meantime it is necessary to remark that the two Classes Reptilia 

 and Aves are of very unequal rank, as far as the structural 

 characteristics which mark them out are regarded. The Birds 

 constitute a very homogeneous group, the different orders of 

 which are chiefly remarkable for structural features associated 

 with minor differences of habit, but the Reptiles are a very 

 heterogeneous group, and the various families, into which the 

 living orders of Chelonia (Turtles), Lacertilia (Lizards), Ophidia 



