LIASSIC FORMATIONS. 95 



hind part of the frontal. The margins of the sagittal suture usually rise into a low ridge, 

 which is continued upon the occipital part (7') of the parietal. Of this part (PI. XXVI, 

 fi°\ 1, 7') the surface on each side of the mesial ridge is feebly concave, almost flat, laterally 

 overlapped by the mastoids (8") ; it seems to rest upon, without sutural junction with, the 

 superoccipital (3). The postero-lateral extension of the parietal (PL XXVI, fig. 1,7a;) 

 curves down beneath the mastoid (s) to within a third part of the lower end of that bone, 

 contributing therewith to the upper and lateral parts of the broad occipital surface. 



The mastoid (8,8', 8") is a large and strong triradiate bone, the rays inclining forward 

 from the outwardly obtuse centre or body forming the prominence at each postero-lateral 

 angle of the skull. The upper and inner ray or branch (PI. XXIII, fig. 1, 8") is three- 

 sided, one facet looking upward, a second backward, the third forward and outward, contri- 

 buting, with the parietal, to the outer and hinder wall of the temporal fossa (t). The angle 

 at which the anterior joins the superior facet is continued forward upon the ridge (8"r), 

 dividing the hinder and upper facets of the parietal bone. The lower mastoid branch 

 (PL XXVI, fig. 1, 8) is a broader plate, smooth, and almost flat externally, forming the 

 upper sides of the occipital region, articulating outwardly with the prosquamosal (27') and 

 tympanic (28), and below with the pointed process of the pterygoid (24), wedged between 

 the mastoid and tympanic, and here overlapped by the paroccipital (4) in its way to abut 

 against the tympanic. The outer branch of the mastoid, smooth and subconvex outwardly 

 (PI. XXIV, fig. 1, 8'), extends forward to form the hinder half of the upper zygoma, 

 overlapped by the post-frontal (Pis. XXIII, XXIV, figs. 1, 12), and articulated along its 

 lower border with the broad sclerodermal plate ('prosquamosal/ 27') ' occupying the 

 interval between the upper and lower zygomata. 



The tympanic is abruptly divided into an upper auditory or proper tympanic portion 

 (PL XXVI, fig. 1, 28) and a lower articular portion (ib., 28')- The former is a narrow, 

 subcompressed, outwardly subconvex, bony piece, and is wedged between the mastoid (8) 

 and prosquamosal (27') ; its hind or mesial border extends from the mastoid junction to 

 articulate with the pterygoid (24) ; the outer or lateral border is smoothly rounded 

 and concave, forming more than the hinder half of the auditory meatus (?«). The suddenly 

 expanded, thick, articular portion (28') joins the pterygoid (24) mesially and the zygomatic 

 (27) laterally, then descends obliquely backward, for an extent equal to the auditory 

 portion, to thicken and terminate in the surface for the articular element of the mandible. 

 This surface is obliquely suboval, convex from before backward, slightly concave trans- 

 versely at its fore part. 



The hyoid, or haemal arch of the second cranial segment, is represented by a basihyal 

 and by a pair of rib-like bones (PL XXV, fig. 3, 38), homologous with the thyrohyals 



1 This osseous plate is described in my "Report on British Fossil Eeptiles," 'Reports of British 

 Association,' 1839, 8vo, p. 9, as the "squamous element of the temporal bone;" it is analogous therewith 

 but not " homologous." 



