‘ 
~Apr.1903. NortH AMERICAN PLESIOSAURS—WILLISTON. 31 
as the splenial (presplenial of Baur), the prearticular (angular of 
Baur), and the coronoid. The identity of the sf/enza/ is assured. It 
has been dislodged upward slightly, disclosing the narrow Meckelian 
groove, which terminates in an orifice at the proximal end of the 
splenial. The bone ends posteriorly below the coronoid eminence. 
Anteriorly it broadens so as to cover all but the upper inner part of 
this surface, uniting with its mate to form the symphysis. From 

FiG. 5, 
Left mandible tosymphysis of Dolichorhynchops osborni. art., articular; fra., prearticular; 
sur, Surangular; cor., coronary; sf., splenial. Compare also PI. II. 
below, the thickened bone forms the inner part of the symphysis for a 
short distance forward, at least. How far it extends can not be deter- 
mined, as it gradually becomes thinner and disappears from view. 
(eee FAs.) : 
The bone which I determine as the coronoid is most pecuhar, 
remarkably unlike that in any other animal which I know. It is a 
long, slender, flattened, trihedral bone, extending far forward, and 
like the splenial, meeting its mate in the median symphysis. It 
extends as far back as the end of the dentary, along its inner side, to 
the most elevated part of the surangular, where it 1s thin and spatu- 
late. It follows the inner margin of the dental border of the dentary, 
apparently at least as far as the middle of the symphysis. At the 
beginning of the symphysis with its mate, the bone is somewhat tri- 
angular in cross-section, with its thin margin below touching the 
splenial; the mesial surface is in contact with that of its mate, while 
the upper surfaceis narrow. On one side the bone, while still retain- 
ing its proper relation with that of the opposite side, has been par- 
tially dislocated from the mandible, so that there can be no question 
of its morphological relations to the contiguous elements. * 
Between the splenial and the coronoid, on the inner side, is 
seen a narrow, thin bone, corresponding quite to the dermogenous 
portion of the articular in the turtles and Rhynchocephalians, that is, 
* This peculiar relation of the coronoid is well illustrated in fig. 13, p.476, vol. xxxvii, of the 
Quarterly Journal of the Geological Society, in Prof. Sollas’ article on ‘‘A New Species of 
Plesiosaurus from the Lower Lias of Charmouth.’’ 
