190 REPTILIA. 



ORDER 6. SQUAMATA. 



The lizards and snakes are of comparatively recent origin, 

 none being known with certainty to date back beyond the 

 Cretaceous period. The hinder region of the skull is degenerate, 

 the lower temporal arcade always, and the upper temporal arcade 

 frequently wanting, so that the quadrate is loosely fixed to the 

 cranium ; the palate also, though primitive in the arrangement 

 of the internal nares, exhibits remarkably large vacuities, and 

 the palatines exclude the pterygoids from articulation with 

 the vomers. The vertebral centra are well-ossified, usually 

 proccelous; and intercentra (hypocentra) or intervertebral 

 wedge-bones are rarely observed. 



The earliest known fossils commonly ascribed to the Squa- 

 mata are fragmentary jaws and procoelous vertebrae from the 

 English Purbeck Beds (Macellodus brodiei), which much resemble 

 those of pleurodont lizards but are too imperfect for discussion. 

 The first satisfactory skeletons, of Cretaceous age, are all refer- 

 able to aquatic members of the order and seem to represent two 

 extinct sub-orders. 



Sub-Order 1. Dolichosauria. 



One of the Cretaceous sub-orders comprises small aquatic 

 animals remarkably snake-like in shape, but with the limbs and 

 their supporting arches completely formed. In allusion to their 

 elongate proportions they are termed DOLICHOSAURIA. The 

 vertebrae are proccelous and resemble those of snakes in being 

 articulated both by zygapophyses and by a zygosphene-zygan- 

 trum arrangement. The typical genus and species, Dolicho- 

 saurus longicollis, from the English Chalk, must have been an 

 animal about 07 m. in length, but only one satisfactory specimen 

 is known. Its head is relatively small and the dentition is 

 pleurodont, the teeth being simple obtuse cones. It has about 

 <)0 presacral vertebrae, of which no less than 17 are in advance 

 of the scapular arch and may thus be termed cervicals. The ribs 

 are hollow. There are the usual two sacral vertebrae, but these 

 are not anchylosed. The limbs are imperfectly known. The 



