LIASSIC ICHTHYOSAURS. 53 



(Tab. XXII, 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 

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

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

 overlapped by the post-frontal (Tabs. XIX, XX, 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 

 (Tab. XXII, 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 (Tab. XXI, fig. 3, 38), homologous with the thyrohyals 

 in the Crocodiles, which elements they resemble in their small relative size, but are of more 

 simple structure. Each is feebly and regularly bent, the convexity (in the petrified skull) 

 turned towards each other, converging forward to their junction with the small flattened 

 basihyal, which junction seems to have been by much ligamentous tissue. I noted in a 

 fossil cranium of a full-sized Iclitliyosaurm communis that the hind ends were 4f inches 

 apart while the fore ends were but \\ inch apart. The total length of fche bone was 

 4 inches 2 lines ; the breadth of the hind end was \ an inch, that of the mid-part 5 lines. 

 In the Ich. loncModon (Tab. XXVII, fig. 5) each thyrohyal is a fifth part the length of the 

 mandible. The stylohyals appear to have retained their fibro-cartilaginous or cartila- 

 ginous tissue, and have consequently disappeared. 



The presphenoid (Tab. XXI, fig. 1, 9) is a long, slender, trihedral bone, broadest where 

 it joins, and commonly coalesces with, the basisphenoid (5) and, along the narrower part, 

 with the two lower sides, converging to a median obtuse angle. It divides the long and 

 narrow, pear-shaped interpterygoid vacuities («,«). 



Of the orbitosphenoids I have no exact knowledge ; they may not have been 



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 

 bnt not homologous. 



