286 COMPARATIVE ZOOLOGY OF CHORDATES 



separated by a joint, which allows the head to move. The 

 articular facets belonging to the skull which take part in this 

 joint are the condyles. In Stegocephalia there are three such 

 condyles, formed by the two exoccipitals and the basioccipital. 

 In higher forms, as will be seen, the number of condyles may 

 be reduced to one or to two, according as to whether the 

 exoccipitals or the basioccipital (respectively) drop out of 

 sharing in the joint. 



In the most primitive reptiles such as Seymouria, the 

 covering of dermal bones is complete, and differs from the 



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m, 



Fig. 133. — Seymouria : dorsal view of a skull. (Drawn from a cast.) 



condition of the Stegocephalia only in that there are no grooves 

 for lateral-line canals. As the nature of the roof of the skull 

 is of the greatest importance in regard to classification in the 

 reptiles, it is necessary to consider a few of the relations which 

 the membrane-bones bear to underlying structures. The 

 more median membrane-bones, such as the nasals, frontals, and 

 parietals, overlie the brain-case directly, and form its roof. 

 But the more lateral membrane-bones of the skull-roof, such 

 as the postorbital, supratemporal, and squamosal lie over the 

 auditory capsules. The auditory capsule, formed by the 



