NORTH AMERICAN PLESIOSAURS 721 



Mandible. — The structure of the mandible is, for the first time, 

 completely made out in a specimen (No. 1125) in the Yale Museum. 

 Although the specimen belongs to the closely allied, possibly identical 

 genus Polycotylus, it may be described here. The suture between the 

 articular and surangular, never before determined, is here clearly 

 shown, running obliquely downward and forward from just in front 

 of the cotylus to connect with the angular-surangular suture beneath 

 the proximal end of the prearticular (fig. 6). In the earher of the 

 cited papers I recognized the differentiation of this element in front 

 of the articular, clearly homologous with the 'splenial' of the turtles, 

 and gave to it the name prearticular. Baur, who had previously 

 recognized this new element in the reptihan mandible unfortunately 

 took for the type of structure that of the turtles, and changed the 

 names of the other elements to conform thereto, giving to the splenial, 

 as the name was originally applied, the name presplenial. This 

 confusion was pointed out in a later pubHcation by me, but with- 

 out mentioning the fact that I had previously proposed the term 

 prearticular for the newly discovered element.^ Kingsley later, in 

 reviewing the mandibular structure proposed for the same element 

 the term dermarticular. In this separation of the prearticular from 

 the articular the plesiosaurs show certain relationships with the 

 turtles, but not important ones, since a Hke condition will probably 

 be found in most of the early reptiles. The great elongation of the 

 coronary, and its union in a median symphysis is the most striking 

 characteristic of the plesiosaur mandible. 



Hyoids. — Under each skull are preserved in perfect condition, 

 and in undisturbed positions the hyoids. They lie below the concave 

 lateral pterygoid plates, the anterior end reaching nearly as far for- 

 ward as the hind end of the interpterygoidal opening. The bones of 

 skull No. 2 (fig. 4) are a little less slender than those of skull No. i. 

 The posterior end is rounded, rod-Hke, and the mesial border is the 

 thinner and more concave one. The hyoids have hitherto been 

 unknown, so far as I am aware, in the plesiosaurs. 



Vertebrae. — The atlas and axis are united with each other and 

 with the occipital condyle in the type specimen. They resemble very 

 closely the same bones in T. osborni. The axial rib is firmly attached. 



I Field Mus. Publ., No. 73, p. 30. 



