532 Richard Owen, Esq., on the 



The only parts of the occipital bone visible in the present head are the 

 superior extremity of the supra-occipital, where it is joined to the posterior 

 point of the bifurcation of the parietal, and the point of the right lateral or 

 exoccipital, where it abuts against the suture between the transverse process 

 or fork of the parietal and the tympanic bone. 



The force which has crushed the head has fractured the parietal just at its 

 point of bifurcation, and the strong transverse forks are separated from each 

 other and from the median or ordinary part of the parietal from which they 

 were continued ; these fractures are also accompanied with a slight disloca- 

 tion of the respective parts. The natural form of the parietal bone in the 

 Piesiosaurus is, however, well displayed in the head figured by Mr. Cony- 

 beare, in the 2nd vol. of the 2nd Series of the Society's Transactions, (PI. 

 XIX. fig. 2.); and the oblique sigmoid suture uniting the extremity of the 

 parietal process to the tympanic bone is also indicated in that figure at 

 about half an inch from the point of bifurcation. In the PL Macrocephalus 

 the transverse processes of the parietal are relatively stouter and longer 

 than in the Plesiosaur there figured, or in the Pi. Hawkinsii. That on 

 the right side in the present specimen forms the most prominent part of the 

 bones of the head. The normal portion of the parietal bone, anterior to the 

 bifurcation, presents a distinct division into two lateral moieties by a sagittal 

 suture. The sides of this suture, or rather harmonia, for it is not denticu- 

 lated, are deep, being formed by the apposition of two ridges of bone which 

 rise and meet to form the longitudinal intermuscular crest which traverses the 

 middle line of the upper surface of the head in this, as in other species of 

 Plesiosaur. Two inches anterior to the bifurcation of the parietal the fora- 

 men which characterizes the parietal in many LacertineSauria is plainly indi- 

 cated by the moiety of its circumference which is due to the left parietal ridge, 

 which alone is here preserved. The long diameter of this vascular foramen 

 is three lines. 



The coronal suture seems here to divide the parietals from the frontals im- 

 mediately anterior to the above foramen, which thus belongs entirely to the 

 parietals, as in the genera Monitor, Lacerta proper, and others; while in the 

 Jchthj/osaurus the corresponding foramen is placed directly upon the coronal 

 suture, as in the genera Iguana and Stellio. 



The parietals slope away immediately from the median ridge, indicating 

 the extent of the strong temporal muscles which were only separated on the 

 top of the head by the thickness of this ridge. In this structure we find the 

 Plesiosaurus resembling the lacertine Iguana among existing Saurians, and 

 diireri)ig from the Crocodiles and Gavials, which have the temporal muscles 





