ApR. 1903. NORTH AMERICAN PLESIOSAURS—WILLISTON. 1g 
nearly undisturbed condition. There are fourteen in the ring with 
beveled and imbricated contiguous margins, in texture, size and 
position very much lke the corresponding bones of the mosasaurs. 
The pupillary opening measures about thirty millimeters in diameter, 
and the entire diameter of the ring is about seventy millimeters. The 
occurrence of sclerotic plates in the plesiosaurs has long been known. 
I described them in Cimoliasaurus snowti in 1890, and Owen many 
years earlier (Fossil Reptilia of the Liassic Formation, p. 10) said: 
‘‘In both orbits some of the thin sclerotic plates of the eyeball are. 
preserved ; this is the first specimen in which I have had evidence of 
their structure.” 
The juga/ is a small element intercalated between maxilla, post- 
orbital and squamoso-prosquamosal. The suture separating it from 
the maxilla runs nearly parallel with the lower border of the bone. 
In its posterior third this suture is very distinct; it seems to be con- 
tinued forward to attain the margin of the orbit at its lower posterior 
part. Above, the bone is distinguished from the postorbital by a 
nearly parallel suture; behind by a nearly transverse suture from the 
squamosal. On the right side, the jugal had been separated from the 
other bones by maceration; its relations, therefore, are positively 
indicated. The bone terminates about twenty millimeters before the 
posterior end of the maxilla. On the inner side, just back of the 
rounded orbital margin, the bone articulates by a flattened surface, 
about the size of one’s finger-nail, with the ectopterygoid. The bone 
is pierced on its outer surface by three or four small zygomatic 
foramina. 
The broad, triradiate element, variously considered as being com- 
posed of, or the homologue of, the squamosal and mastoid by Owen*, 
the squamosal and supratemporal by Andrews?, the squamosal and 
prosquamosal by Owen and Baur, the supratemporal and supramas- 
toid by Copet, the squamosal, supratemporal and quadratojugal by 
Woodward§, differs materially in its structure from that described or 
figured in other plesiosaurian skulls, in that the element, or elements, 
whatever they are, articulate proximally with the maxilla, as well as 
. the postfrontal and jugal. Posteriorly, the suture separating the bone 
* Trans. Geol. Soc. Lond. (2), v, pt. iil, pl. xlv, 1840. 
+ Quart. Journ. Geol. Soc. Lond. lvii, 249, 1896. 
¢{ Proc. Amer. Phil. Soc. xxxiii, 110, 1894. Cope, in his essay on the posterior cranial arches itr 
the Reptilia (Trans. Amer. Phil. Soc. 1892), reaches the conclusion that the lower temporal bar of 
the Crocodilia, SAhenodon, etc., corresponds with the zygomatic arch of the mammalia, and there- 
fore suppresses the term ‘‘squamosal.’’ The squamosal—so-called—in the Reptilia he calls the 
supramastoid, absent in the lacertilia and other forms. 
§ Vert. Paleont., f. 116 A, 1898. 
