8AUR0PSIDA. 



CHAPTER XXVII. 

 CLASS m. REPTILIA. 



We commence now the second great primary division of 

 the Vertebrata, namely, that of the Sawopsida, comprising 

 the Reptiles and the Birds. These two classes, though very 

 unUke in external appearance, are united by the following 

 characters : There are never at any period of life giUs or bran- 

 chiae adapted for aquatic respiration ; the red corpuscles of 

 the blood are nucleated (Fig. 99, h, c) ; the skull articulates 

 with the vertebral column by means of a single articulating 

 surface or condyle ; each half of the lower jaw is composed 

 of several pieces, and is jointed to the skull, not directly, but 

 by the intervention of a special bone (the so-called " quadrate 

 bone " ). 



These being the characters by which, among others, Rep- 

 tiles and Birds are collectively distinguished from other Ver- 

 tebrates, it remains to see what are the characters by which 

 the Reptiles are distinguished, as a class, from Birds. In all 

 ReptUes the blood is cold — that is to say, very slightly warm- 

 er than the temperature of the external medium in which they 

 live. The integument secretes scales, with or without bony 

 plates, but in no case do the integumentary appendages take 

 the form of feathers. The heart consists of two auricles, and 

 a ventricle, which in most is partially divided into two cham- 

 bers by an incomplete partition, and in a few is completely 

 divided. In any case, however, more or less of the impure 

 venous blood is mixed with the pure arterial blood which cir- 

 culates over the body. There is no division between the cavi- 

 ties of the thorax and abdomen, and the lungs are not con- 

 nected with air-sacs placed in various parts of the body. The 

 limbs may be wanting, or rudimentary, but in no case are the 



