192 CHARACTERS OF VERTEBRATE ANIMALS 



External Characters. — The flattened triangular head, with 

 blunt forwardly-directed apex, is separated by a short ill-marked 

 neck from the long rounded trunk, and this again passes into 

 a very long slender tail. The shape is well adapted to the 

 darting movements by which the animal makes its way through 

 heather and grass. The limbs are short and primitive-looking: 

 they are divided into the same regions which we have found 

 to be typical for Mammals (p. 24) and Birds (p. 140); i.e. 

 upper arm, fore-arm, and hand for the front-limb, corresponding 

 to the thigh, lower leg, and foot in the hind-limb. There 

 are five fingers and five toes, all ending in claws, and the 

 shortest digits in their respective limbs are the thumb and great 

 toe. The body is not lifted well off the ground by the limbs, 

 and this is not because these are short, but owing to their 

 "sprawling" arrangement, knee and elbow projecting outward, 

 instead of being brought well below the trunk, as in a dog, cat, 

 or other Mammal. 



Returning to the head, the flatness of which as compared with 

 a Bird is due to the relatively small brain, we note a large mouth 

 extending far back from the front end of the snout, near the tip of 

 which can be seen the two small nostrils. Further back come the 

 good-sized eyes, which, as in a Bird, are provided with a nictitat- 

 ing membrane (p. 140), as well as with upper and lower eyelids. 

 Some little distance behind either eye, not far from the angle of 

 the mouth, is an auditory aperture, devoid, as in Birds, of an 

 external flap or pinna. Here, however, the external passage of the 

 ear (external auditory meatus), which leads down to the tympanic 

 membrane, is excessively short, so that the membrane is brought 

 close to the surface, and can readily be seen without dissection. 



The only other aperture of importance is that of the cloaca 

 (p. 146), a transverse cleft on the under side of the trunk where it 

 passes into the tail. 



While Mammals are distinguished by the possession of hair, 

 and Birds by their feathers, Reptiles have an exoskeleton of 

 scales, which, like hairs and feathers, are products of the outer 

 layer of the skin, or epidermis. Bony plates, or bony scutes 

 formed in the deeper part of the skin (dermis), may be present 

 as well. The scales of the Sand Lizard are very obvious struc- 

 tures with a definite arrangement. Those on the head are com- 

 paratively large, and it has been found convenient to give some 



