OSTEOLOGY OF AMERICAN PERMIAN VERTEBRATES 413 



mandibular symphysis, the splenial takes part in its union." But, 

 this symphyseal union of the splenials in such forrns is doubtless 

 due to the recession of the symphysis, rather than to the retention 

 of a primitive character, since in the alligator and crocodile, with a 

 short symphysis, the splenials do not meet, while in the gavial, with 

 a long symphysis, they do. 



Fig. 13. — Dimetrodon incishus Cope: A, left mandible, outer side; B, inner 

 side of same; cor, coronoid; d, dentary; sa, surangular; an, angular; art, articular; 

 pa, prearticular; sp, splenial. 



Not only is the symphyseal union of the splenials a primitive 

 character, as was first stated by me in 1911^ and reaffirmed the 

 following year, but, unlike modern reptiles, it enters more or less 

 into the outer surface of the mandible, as I have figured it in various 

 cotylosaurs {Limnoscelis, Labidosaurus , Captorhinus). These primi- 

 tive characters may be summarized as follows: 



The primitive mandible of reptiles is composed of seven distinct 

 bones, the articular, prearticular, angular, surangular, coronoid, 

 splenial, and dentary. The coronoid is elongated, extending along 



■Williston, American Permian Vertebrates, p. 30. 



