456 BRITISH FOSSIL REPTILES. 



The parietal bone (7) is rmich compres-sed, and develops a sharp and rather lofty 

 median crest behind the forannen (p), which crest divides the temporal fossae {t, '). 

 Behind this crest the parietal bone expands transversely, and assumes a tri-radiate 

 form, the two transverse rays uniting with the mastoids (8, 8). These are very 

 powerful bones, bounding the outer and back part of the temporal fossae ; they 

 are smooth and slightly convex above, rough and slightly concave at the back part 

 near the angle, where a surface is thus formed for the attachment of some powerful 

 muscle. The part of the mastoid which curves forward from the angle to form the 

 back part of the zygomatic arch becomes compressed, and terminates above in a 

 ridge (r). The substance of the mastoid is extensively excavated, apparently for 

 the upper part of the acoustic chamber. 



The frontal bone (ii) is overlapped behind by the parietal, and appears to 

 have been divided by a median " harmonia," or smooth suture ; the receding 

 halves of the frontal behind, as they pass beneath the parietal, form the fore part 

 of the foramen parietale. The back part of the foramen is formed by a notch 

 in the fore part of the single and undivided parietal. The canal from the foramen 

 extends obliquely downward and backward. The long diameter of the foramen is 

 1 inch ; the breadth of the back part of the cranium is 16 inches ; the breadth of 

 the back part of each temporal fossa is 6^ inches. The power of the muscles acting 

 upon the lower jaw must have been very great. 



A portion of a symmetrical bone, 10 inches long, which formed the upper 

 median part of the face, anterior to the orbits, represents part of an undivided nasal 

 bone (15), and shows that bone to have been long, narrow, straight longitudinally, 

 convex transversely above, as if the upper part of the face had been traversed by a 

 low, obtuse, median rising. 



In most of these characters may be discerned a closer affinity to the Plesio- 

 sauroid than to the Crocodilian type. 



The expanse of the temporal fossae equals that in the Plesiosauria and Teleo- 

 sauria, but no species of the latter genus of Crocodilia has presented the " foramen 

 parietale," whilst it is a constant character in the Plesiosauria, Ichthyosauri, and 

 Lahyrinthodo7itia ; many of the modern lizards also present the same foramen. 

 The portion of the upper maxillary bone, figured of the natural size at fig. 2, PI. 14, 

 shows the same obliquity of the separate sockets of the teeth as exists in those at 

 the fore part of the bone in certain Plesiosauri, and the small separate foramina 

 (o, o), at the inner and back part of the large alveoli, which had been perforated 

 by the summits of the successional teeth, are of plesiosauroid character. I 

 have seen portions of jaws oi Plesiosaiirus meyacephahis in wliich the appearance of 

 a double row of teeth was caused by the length of the protruding summits of tlie 

 new teeth before they displace the old when they are pushed, causing absorption 

 of the intervening osseous bar, into the large sockets of the teeth they replace. 



