412 



S. W. WILLISTON 



the dentary, surangular, and prearticular or angular, and also the 

 splenial when present. It never extends farther forward than 

 the posterior part of the dental series. In 

 the same plesiosaurian mandible in which I 

 recognized a separate prearticular, there was 

 "a long, slender, flattened, trihedral bone, 

 extending far forward along the alveolar 

 margin, meeting its mate in the median 

 symphysis" (Fig. i6), which I was forced to 

 call the coronoid, though it was utterly unlike 

 anything that had previously been recognized 

 as the coronoid. Only within recent years has 

 this peculiar structure of the coronoid been 

 recognized in other plesiosaurs, by Andrews 

 and Linder; doubtless it is characteristic of 

 the order, testifying to its primitive origin. 

 This elongate form of the coronoid is now 

 known to be characteristic of all primitive 

 reptiles. In these reptiles it lies along the 

 inner alveolar margin of the mandible, ex- 

 tending toward the symphysis, but entering 

 into union with its mate only in those reptiles 

 having a long mandibular symphysis. Its 

 posterior end occupies the position of the 

 coronoid of later reptiles, that is, articulat- 

 ing with the anterior superior part of the 

 surangular, chiefly on the inner side of the 

 mandible, and forming the anterior margin of 

 the meckelian orifice. It articulates with the 

 prearticular below; it is always narrow. 



The splenial in all modern reptiles, when 

 , ^ y / present, is a thin bone covering more or less 



/5 # the inner side of the mandible anteriorly and 



^'^ the meckelian groove, extending rarely as far 



forward as the symphysis; it is not visible on the outer side of the 

 mandible, or, if so, only to a very slight extent. As I stated some 

 years ago,^ "in all or nearly all reptiles having a greatly elongated 



^ Williston, Jour. Geo!., XVI (1906), 6. 



^ 



