2 9 S 



CLASSIFICATION OF VERTEBRATES. 



(lacking in chameleons) is usually well developed, and affords a 

 .support for the ventral ends of the clavicles. In many turtles 

 it, together with the clavicles, enters into 

 the formation of the plastron. 



The skull of recent reptiles differs in 

 many respects from that of existing am- 

 phibia ; but when the fossil groups are con- 

 sidered, the distinctions largely disappear, 

 the skulls of stegocephalans and thero- 

 morphs being strikingly similar. In these 

 lower reptiles the top of the skull forms a 

 continuous roof above the attachment of 

 the jaw muscles ; but in other groups gaps 

 or vacuities may occur, so that these mus- 

 cles are exposed from above. These va- 

 cuities or fossae exhibit the following vari- 

 ations : (i), between the parietals and 

 postorbitals (supraternporal fossa) ; (2), 

 between postorbitals and squamoso-jugal 

 (infratemporal fossa) ; (3), between the 

 post-temporal and the exoccipital and op- 

 isthotic (post-temporal fossa) ; (4), the 

 line of bones (arcade) between i and 2 

 may be interrupted, producing one large 

 temporal fossa; (5), the squamosojugal arcade may be discon- 

 tinuous. 



As a rule the cartilage of the primordial cranium is largely 

 replaced by bone, the ethmoid and parts of the sphenoid alone 

 rjeing incompletely ossified. Except in a few theromorphs there 

 is but a single occipital condyle, which is either formed by the 

 basioccipital alone or with the participation of the exoccipitals. 

 Either basi- or supraoccipitals may be excluded from the for- 

 amen magnum. In the ear region a fenestra rotunda appears ; 

 of the otic bones the prootic is always distinct, the epiotic is 

 fused with the supraoccipital, while the opisthotics (free in 

 turtles) are usually united to the exoccipitals. 



While in some the brain extends forwards between the orbits, 

 it frequently does not reach so far forward, and the orbits them- 



FiG. 294. Pectoral 

 girdle and sternum of 

 lizard, Lceinanctus lou- 

 gipes, after Parker, c, 

 coracoid ; ct, clavicle ; 

 <?, episternum ; g, glenoid 

 fossa ; p, procoracoid ; 

 ?', rib ; s, scapula ; st, 

 sternum ; x, xiphister- 

 num. Cart'lage dotted. 



