CHARACTERS OF REPTILIA. 393 



DIVISION II. SAUROPSIDA. 



CHAPTER LXII. 

 CLASS III. REPTILIA. 



THE second great division of the Vertebrate Sub-kingdom, 

 according to Huxley, is that of the Sauropsida, comprising the 

 true Reptiles and the Birds. It is, no doubt, at first sight an 

 almost incredible thing that there should be any near bond of 

 relationship between the Birds and the Reptiles, no two classes 

 of animals being more unlike one another in habits and ex- 

 ternal appearance. It is, nevertheless, the fact that the Birds 

 are more nearly related to the Reptiles than to any other class 

 of the Vertebrata, and it will shortly be seen that many affini- 

 ties and even transitional forms are known to exist between 

 these great sections. The Reptiles and Birds, then, may be 

 naturally included in a single primary section of Vertebrates, 

 which may be called Sauropsida after Huxley, and which is 

 defined by the possession of the following characters : At no 

 period of existence are branchiae, or water-breathing respiratory 

 organs, developed upon the visceral arches; the embryo is 

 furnished with a well-developed amnion and allantois ; the red 

 corpuscles of the blood are nucleated (fig. 122, , c); the skull 

 articulates with the vertebral column by means of a single 

 articulating surface or condyle ; and each half or " ramus " of 

 the lower jaw is composed of several pieces, and articulates 

 with the skull, not directly, but by the intervention of a pecu- 

 liar bone, called the " quadrate bone," or " os quadratum " 

 (fig. 153). 



These being the common characters of Reptiles and Birds 

 by which they are collectively distinguished from other Verte- 

 brates, it remains to inquire what are the characters by which 

 they are distinguished from one another. The following, then, 

 are the characters which separate the Reptiles from the Birds : 

 The blood in Reptiles is cold that is to say, slightly warmer 

 than the external medium owing mainly to the fact that the 

 pulmonary and systemic circulations are always directly con- 



