562 TEXTBOOK OF ZOOLOGY 



External Structure 



The horned lizards are typical iguanid lizards, with broad, flat 

 bodies covered with horny, strongly keeled scales and spines. There 

 is at least one row of spines in a marginal fringe at the lateral 

 edges of the belly. Erect, scattered spines of various shapes and 

 sizes are apparent on the animal's back. The body is devoid of 

 a dorsal crest but is covered with small granular scales and with 

 four rows of enlarged, sharply pointed spines. The ventral parts 

 are covered with small, smooth, light-colored, rectangular scales. 



The body of the animal is divided into three well-defined regions : 

 head, neck, and trunk. The trunk bears two pair of well-developed 

 linibs and tapers posteriorly to a broad, short tail. 



The head is short and the muzzle descends steeply in profile, but 

 is not separated from the front by a conspicuous angle. It is 

 covered with small scales, and is bordered posteriorly by a row of 

 osseous spines. The most anterior structures are paired, small 

 nostrils. These are small rounded apertures situated one on each 

 side of the snout, a short distance from its anterior end. Just 

 posterior to the nostrils are the eyes. They are situated one on 

 each side of the head about midway between the nostrils and the 

 tympanic area. Each eye is guarded by a short, thick upper eyelid 

 and a thinner lower lid. The lower lid covers most of the eye when 

 that organ is closed. Within the eyelids and attached to the anterior 

 comer of the eye is a thin, transparent nictitating membrane which 

 closes backward over the eye. Above each orbital socket is a bony 

 structure which forms a posterior superciliary angle by being pro- 

 duced into a short postorbital horn. Behind the eye and a little 

 posterior to the mouth is the auditory aperture. A thin tympanic 

 memhrane is stretched over it. Above the auditory aperture and be- 

 tween it and the eye are three temporal horns. They form the post- 

 lateral border of the head. The occipital horns mark the dorsoposte- 

 rior boundary of the head. The gape of the large mouth begins at 

 the snout and extends posteriorly to within a short distance of the 

 auditory aperture. The lower labial scales vary from the small 

 rounded anterior ones to the prominent, acute posterior spinelike 

 scales. 



The cervical region is thick and stout but well differentiated. A 

 transverse gular fold is present on the ventral surface of the neck. 



