Apr. 1903. NOrtH AMERICAN PLESIOSAURS—WILLISTON. 29 
the parietal, is elongate oval in shape, slightly convex in both direc- 
tions, and turned obliquely inward posteriorly, so that the two bones 
when in place formaV. The surface looks forward and upward, and 
joins a projecting sutural surface of like shape on the parietal bone. 
Manoipte.—F rom the exterior of the mandible four elements are 
visible, arranged much as in the crocodile or Sphenodon. The dent- 
ary extends far back along the upper border, quite to the top of the 
coronary eminence. Thence its suture runs obliquely to a little 
beyond the posterior end of the symphysis on the lower border. The 
element back of this on the upper border is doubtless the surangular, 
separated from the angular below by a suture placed very much as it 
is in the crocodile, beginning at the extreme posterior end of the 
mandible. The bone extends anteriorly as an elongated point between 
the dentary above and the angular below. ‘The suture separating the 
element from the articular cannot be made out. The two, united, 
agree quite with the element described by Guenther in Sphenodon, and 
as seen in a specimen fifty-eight millimeters in length before me. [| 
distinguish in this mandible, as did Guenther in his, only four elements 
—the dentary, which reaches far back; the coronoid, a flat triangular 
bone occupying its usual place; the articular, inclusive of the 
surangular; and the angular. Baur* describes five elements in a 
Sphenodon skull fifty-six millimeters in length. The articular he 
restricts to a small nodule or disk of bone, similar to that of the 
turtles, forming the articular surface; the surangular, the bone 
before the cotylus, which he indicates as separated by a suture; the 
angular he considers to be the inner prolongation of the bone which 
reaches to the coronoid. The slender bone usually called the angular 
he believes to be the splenial; while the bone usually called the 
splenial (presplenial, Baur) in the crocodile and lizard he believes to 
be wanting in the Sphenodon, as it usually is in the turtles. The 
small ossification which he finds in the cotylus’ of the young 
Sphenodon, similar to the element in a like place in the Zestudinata, he 
assumes to be present in all reptilian mandibles, but is obliterated in 
the adult skull by the anchylosis of the suture. I certainly do not 
- find such a bone in the Sfphenodon mandible before me, nor could Guen- 
ther distinguish such an element. He believes then, that the element 
usually considered the articular, is in reality composed of two bones — 
a chondrogenous articular part and a dermogenous anterior prolonga- 
tion. This is probably true, but I do not see the necessity of chang- 
ing the names of the other anterior elements and of calling this 

* American Naturalist, 1891, 
