320 CLASS REPTILIA. 



tilia tlie supratemporal arcade alone is present and in Ophidia 

 both arcades are absent. 



In many reptiles, e.g. RhyncJwcepkalia, Crocodilia, Lacertilia 

 and some Chelonia the parietal sends out a process which reaches 

 the squamosal : this is the parieto-squamosal or posterior 

 temporal arcade, which forms the superior boundary of a pos- 

 terior temporal fossa, the lower boundary of which is formed 

 by the parotic process of the exoccipital and periotic bones. 



The palate varies considerably in structure in the different 

 groups, but the pterygoids tend to converge rapidly towards the 

 middle line, instead of being parallel or even divergent as in 

 the Amphibia.* The palatal vacuities may be extensive as in 

 Lacertilia and Ophidia, or there may be a continuous bony 

 palate with secondary palatal plates, as in Chelonia and 

 Crocodilia. A transpalatine (ectopterygoid) connecting the 

 pterygoid and maxilla is present in many forms. 



The lower jaw is composed of five membrane bones, the den- 

 tary, splenial, angular, surangular and coronoid, and of a cartilage 

 bone the articular. Meckel's cartilage often persists through life. 

 In the shoulder girdle there is a scapula and a coracoid which 

 reaches the sternum, when that structure is present ; and 

 clavicles and interclavicles are frequently found. The humerus 

 is provided with one or two condylar foramina in Sphenodon and 

 some extinct forms, and the number of digits varies from two 

 to five, or in Ichthyosauria to even a larger number. 



The pelvis exhibits great variations which are described later 

 under the orders : it may even be mammal-like (some Anomo- 

 dontia) or bird-like {Dinosauria). The ankle-joint is intertarsal 

 and the number and form of the toes vary considerably. 



Central nervous system. — The spinal cord possesses 

 except in snakes cervical and lumbar enlargements and in some 

 extinct forms the .^umbar swelling seems to have been larger 

 than the brain. 



The cerebro- spinal axis is bent at the junction of spinal cord 

 and brain. The cerebral hemispheres (Fig. 177) are small and 

 smooth ; they are largest in the Crocodilia. There are two 

 optic lobes. The cerebellum is a mere strip in snakes and lizards : 

 it is rather larger in Chelonia, and in Crocodilia it consists of a 



* Mr. Lister has called my attention to this character, which has not, 

 so far as I know, been noticed before. 



