REPTILIA. SQUAMATA. 199 



with four short limbs as in lizards, or it may be limb- 

 less, extremely elongated, and passing imperceptibly into 

 the tail. The surface is generally completely covered with 

 overlapping horny epidermal scales, below which bony dermal 

 scutes may be developed. 



The vertebrae are procoelous, rarely amphicoelous. There 

 are no intercentra, and the neural arches are firmly united to 

 the centra. Additional articulating surfaces, the zygosphenes 

 and zygantra, are often developed 1 . The sacrum is formed 

 of two or rarely three vertebrae, or may be wanting as in 

 Ophidia. In the skull an infratemporal arcade forming the 

 lower boundary of the infratemporal fossa is absent, and the 

 quadrate, except in the Chamaeleons, is movably articulated 

 to the squamosal. The palatal vacuities are large and the 

 nares are separate. There is often a distinct parasphenoid. 

 The teeth are either acrodont (i.e. ankylosed to the summit 

 of the jaw), or pleurodont, i.e. ankylosed to the inner side 

 of the jaw. The thoracic ribs each have a single head 

 which articulates with the centrum of the vertebra; while 

 uncinate processes and abdominal ribs never occur. 



A pectoral girdle and sternum may be present, or may be 

 completely absent as in snakes. Except in snakes there are 

 generally four pentedactylate limbs which may either form 

 paddles or be adapted for walking. 



Suborder (1). LACERTiLiA 2 . 



The body is elongated, and as a rule four short pentedacty- 

 late limbs are present, but sometimes limbs are vestigial or 



1 Zygosphenes are extra articulating surfaces borne upon the anterior 

 face of the neural arch ; they fit into corresponding structures, the 

 zygantra, which are borne on the posterior surface of the neural arch 

 of the preceding vertebra. Ordinary zygapophyses always accompany 

 them. 



2 See E. D. Cope, P. Amer. Phil. Soc. vol. xxx. p. 185. 



