Apr. 1903. NortTH AMERICAN PLESIOSAURS— WILLISTON. 55 
The supraoccipitals are visible from the side behind and are not 
covered over by the wing-like expansions of the parietals, as in Dodv- 
chorhynchops. They are paired and separated throughout, as in that 
genus. Below they unite with the exoccipital, and include a part of 
the semicircular canals. Above and in front they join the parietals 
at their posterior margin and not under the roof. The exoccipitals 
are also visible. The paroccipital process is slender, its posterior 
margin thin, joining the supraoccipital in an angle without the small 
excavation which is seen in Dolichorhynchops at this place. 
Lying within the orbit are thirteen thin, bony sc/erotic plates, the 
largest about twenty millimeters in diameter, with somewhat crenu- 
lated margins. The larger number are lying in position, imbricated 
with each other. 
There are sockets inthe mandibles for nineteen or twenty teeth 
on each side. Those in the upper jaws seem to be the same in num- 
ber, though the small posterior ones are so covered by the inferior 
teeth that the number cannot be positively determined. The largest 
teeth implanted in the upper jaw are those of the premaxilla; back 
of these there is but a single large tooth, situated just in front of the 
orbit. The largest teeth of the mandible are the ones corresponding 
to those of the premaxilla; the posterior ones, however, are much 
larger than the corresponding ones of the upper jaw. The anterior 
teeth, especially, are elongate, conical and lightly recurved. All are 
sharply pointed, with the crown, within a half or three-fourths of an 
inch of the socket, finely striated. The largest is that of the pre- 
maxilla just in front of the maxillary suture, which measures fifty- 
three millimeters in length by thirteen in width at the base of the 
crown. ‘The first tooth in the mandible is fully as long, thougha 
little more slender. 
The pteryvgoids are so crowded in between the mandibles and 
maxille that only a portion of them is visible. The posterior part 
joins the quadrate by a stout plate, as in Dolichorhynchops. By the 
sides of the sphenoids the plates are broad and massive, with a 
thinned outer crenulated margin. The ec/opterygo’d unites with the 
jugal and maxilla. Evidently there are elongated interpterygoid 
vacuities, as in Dolichorhynchops. 
The mandible, from the tip of the symphysis to the hind extremity, 
measures 480 millimeters, of which the teeth occupy 320. Its least 
width, near the middle, is 40 millimeters; its greatest width, just back 
of the teeth at the coronary eminence, is 75 millimeters. The length 
of the symphysis is 65 millimeters. The two sides are firmly codssified, 
traces of the suture being visible in the posterior part only. The 
