Apr. 1903. North American Plesiosal rs-»— Willis ion. 19 



nearly undisturbed condition. There are fourteen in the ring with 

 beveled and imbricated contiguous margins, in texture, size and 

 position very much like 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. 

 1 described them in Cimoliasaurus snowii 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 jugal 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 ppsterior 

 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 Andrewst, the squamosal and 

 prosquamosal by Owen and Baur, the supratemporal and supramas- 

 toid by CopeJ, 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 

 :he postfrontal and jugal. Posteriorly, the suture separating the. bone 



♦Trans. Geol. Soc. Load. (2). v. pt, iii. pi. xlv. 1840. 



t Quart. Journ. Geol. Soc. Lond. lvii, 249. 1896. 



% Proc. Amer. Phil. Soc xxxiii, no, 1894. Cope, in his es-;i\ on the posterior cranial arches in 

 he Reptilia (Trans. Amer. Phil. Soc. 1892), reaches the conclusion that the lower temporal har of 

 he Crocodilia, Sphcnodoiiy etc.. corresponds with the zygomatic arch of the mammalia, and there* 

 .-<■ suppresses the term '•squamosal." The squamosal— so-called— in the Reptilia he calls the 

 upramastoid. absent in the lacertilia and other forms. 



S Vert. Paleont., f. 116 A, 1898. 



