256 VERTEBRATE ANIMALS. 
SAUROPSIDA. 
CUA. PUB, X20 Vel, 
CLASS Il. REPTILIA. 
WE commence now the second great primary division of the 
Vertebrata—namely, that of the Sauropszda, comprising the 
Reptiles and the Birds. These two classes, though very unlike 
in external appearance, are united by the following charac- 
ters: There are never at any period of life gills or branchiz 
adapted for aquatic respiration; the red corpuscles of the 
blood are nucleated (fig. 121, 6 c); the skull articulates with 
the vertebral column by means of a single articulating sur- 
face 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, amongst others, Rep- 
tiles and Birds are collectively distinguished from other Verte- 
brates, it remains to see what are the characters by which the 
Reptiles are distinguished, as a class, from Birds. In all Rep- 
tiles the blood is cold—that is to say, very slightly warmer 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 chambers by an in- 
complete partition, and in a few is completely divided. In any 
case, however, more or less of the impure venous blood 1s 
mixed with the pure arterial blood which circulates over the 
body. There is no division between the cavities of the thorax 
and abdomen, and the lungs are not connected with air-sacs 
placed in various parts of the body. The limbs may be want- 
