42 



ZOOLOGY. 



the cavities of the chest and abdomen are not separated 

 by a diaphragm ; the hemispheres of the brain are not 

 united by a corpus callosum. 



On the other hand, reptiles differ from birds in the 

 following particulars : The covering of reptiles consists 

 of horny scales, and sometimes, also, of bony plates, never 

 of feathers ; the fore-limbs never take the form of wings ; 

 the tarsal and metatarsal bones of the hind limb are never 

 anchylosed ; the heart generally contains three cavities, 

 and the blood is cold : the bronchi of the lungs never 

 terminate in air sacs. 



43. Skeleton. The vertebral column varies much in the 

 different groups. Some serpents have as many as three 

 hundred vertebrae, united by a "ball and socket" joint. 

 The tail is generally very long. The limbs, when pre- 



FIG. 14. SKULL OP A VIPER. 

 a, view from above ; &, from below ; c, side view. 



sent, are sometimes used in walking, sometimes in 

 swimming, and in the extinct pterodactyles (Gr. pteron, 

 a wing ; daktulos, a finger) as organs of flight. In this 

 last case, however, the skeleton of the fore-limb had no 

 resemblance to the wing bones of a bird. The serpents 

 and some lizards are destitute of limbs. 



There is a single condyle on the occipital bone, and a 

 corresponding facet on the atlas. There are from four to 

 six pieces in each half of the lower jaw, which is united 



