Apr. 1903. North American Plesiosaurs — Williston. 55 



The supraoccipitah are visible from the sick behind and are not 

 covered over by the wing-like expansions of the parietals, as in Doli- 

 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 sclerotic 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 in the 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 

 1 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, though a 

 little more slender. 



The pterygoids are so crowded in between the mandibles and 

 maxilla; 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 ectopterygoid 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 coOssified, 

 traces of the suture being visible in the posterior part only. The 



