86 MAEINE EEPTILES OF THE OXEOED CLAT. 



The frontals (/., PI. III. figs. 2,2a; PI. VI. fig. 1 ; text-fig. 46) are large, and there 

 may be some doubt whether the bones so called here may not include other elements, 

 though in no case can any trace of sutures be observed. Posteriorly they join the 

 parietals in a transverse suture and, as above mentioned, may or may not form the 

 anterior edge of the pineal foramen. External to their union with the parietals they 

 join the postfrontals for a short distance. In front of tliis, again, they widen out by a 

 gentle curve, forming the roof of the orbit. In this region the upper surface of the 

 two bones is convex from side to side laterally and concave mesially, where they 

 unite with one another in the median line, so that here the roof of the skull is concave 

 from side to side. 



In front of the orbit the frontal attains its greatest width, sending downwards 

 and outwards a process, which terminates in a sutural surface, presumably for union 

 with the prefrontal, unless that element is actually included in the bone here called 

 frontal. In front of the prefrontal process the bone again narrows and is notched by 

 the inner border of the external narial opening. In the middle line as far back as 

 the anterior third of the orbit the middle portion of the frontals is concealed beneath 

 the long overlapping facial processes of the premaxillse, which thus extend far back 

 behind the external nares. On their ventral face (PI. III. fig. 2) the frontals bear on 

 either side a strong ridge or crest continuous posteriorly with the ridges on the 

 parietals, referred to above. The ridges run parallel with one another and separated by 

 only a narrow interval as far as opposite the anterior third of the orbital border, where 

 they bifurcate, one branch running outwards on the preorbital region and helping to 

 form the anterior wall of the orbit, the other continuing parallel with its fellow, on the 

 lower face of that part of the frontal which is concealed by the facial processes of the 

 premaxillse. Between the divergent branches of the ridge there is a deep hollow, 

 where the roofing-bone is very thin. In the orbital region the outer surface of the 

 ridge is concave from below upwards as it passes to the orbital border, thus helping 

 to form not only the roof but also the upper part of the inner wall of the eye-socket. 



The 2^0i<ffro7ifaIs {po.f., PI. VI. fig. 1 ; text-fig. 46) stand out nearly at right angles 

 to the long axis of the skull ; at their inner end they unite posteriorly with the 

 parietals, anteriorly with the frontals. The upper surface of these bones is concave 

 posteriorly, but convex in front, the two areas being separated by a rounded ridge, the 

 continuation outwards of the borders of the divisions of the sagittal crest, where they 

 diverge on either side of the pineal foramen. On the outer end and on the outer 

 portion of the posterior border are sutural surfaces, by which probably the bone joined 

 the postorhtal (p.orb.). The anterior edge of the bone constitutes the upper part of 

 the posterior border of the orbit. 



The squamosal {sq., PI. VI. fig. 1 ; text-figs. 46, 47), perhaps including the supra 

 temporal, is badly preserved in all the skulls of Mitrmwsaurus available for description. 

 It is a triradiate bone ; the upper, somewhat slender arm runs up to the lateral processes 



