200 THE VERTEBRATE SKELETON. 



absent. The exoskeleton generally has the form of horny 

 plates, spines, or scales ; while sometimes as in the Chamae- 

 leons and Amphisbaenians it is absent. In other forms such 

 as Tiliqua and Scincus, the body has a complete armour of 

 bony scutes, whose shape corresponds with that of the over- 

 lying horny scales. 



The vertebrae are precocious, rarely as in the Geckos 

 amphicoelous ; they are usually without zygosphenes and 

 zygantra, but these structures occur in the Iguanidae. The 

 sacral vertebrae of living forms are not ankylosed together, 

 and the caudal vertebrae usually have well-developed chevron 

 bones. 



In the skull 1 the orbits are separated from one another, 

 only by an imperfectly developed interorbital septum, the 

 cranial cavity not extending forwards between them, while 

 the alisphenoidal region is unossified. The premaxillae may 

 be paired or united (Amphisbaenidae), and there is usually 

 an interparietal foramen. There may be a complete supra- 

 temporal 2 arcade bounding the lower margin of the supra- 

 temporal fossa, or the supratemporal fossa may be open below. 

 The quadratojugal is not ossified, and the quadrate articulates 

 with the exoccipital. There is no infratemporal arcade. There 

 is commonly a rod-like epipterygoid 3 (fig. 33, 14) connecting 

 the pterygoid and parietal. 



Teeth are always present, and may be confined to the 

 jaws or may be developed also on the pterygoids and rarely 

 on the palatines ; they are either acrodont or pleurodont. 

 The rami of the mandible are suturally united. 



A pectoral girdle is always present, and generally also a 

 sternum. Clavicles and a T-shaped interclavicle are commonly 

 present, but are absent in the Chamaeleons. 



1 See W. K. Parker, Phil. Trans, vol. 170, 1879, p. 595. 



2 See p. 281. 



:{ Often called the columella cranii. 



