54 BRITISH FOSSIL REPTILES. 



ossified, there is no trace or sign of the lacertian columellar bone near that part of tlie 

 cranium. 



The frontals, or raidfrontals (Tab. XXIII, fig. 1, ii), are small, transversely convex in 

 greater or less degree according to the species, curving in toward the mid-suture in most, 

 and bending outward and downward to the obtuse angle intervening between the nasal 

 (15) and the postfrontals (12). At the hind end of the frontal suture each border curves 

 outwardly to contribute their large share to the 'foramen parietale,' then converging and, 

 in some species {Jch. latifrons, Tab. XXIII), wholly encompassing it ; here, also, as in some 

 other species, the midfrontals distinctly join the postfrontals. In all the species the hind 

 border joins the parietal (7), the fore border the nasal (15). The midfrontals are widely 

 separated from the orbits by the postfrontals and nasals. I have not had clear evidence 

 of their touching the prefrontals, or of the presence of distinct superorbitals. In 

 Icldhyosaurus latifrons (ib., fig. 1) the frontal suture becomes obliterated, and the bone 

 (11) is convex lengthwise as well as across. 



The postfrontals (ib., and Tab. XIX, fig. 1, 12) exceed the midfrontals in size. Each 

 extends from a mesial angle outwards, expanding horizontally, and inclining to form its 

 share of the superorbital border ; thence the postfrontal is continued backward, bending 

 down to join the postorbital (12') and prosquamosal (27'), and contracting to a notched and 

 pointed end, which receives and overlaps the fore end of the zygomatic ray of the 

 mastoid (Tab. XX, fig. 1, 8'). The similarity of character in the postfrontal (12) and 

 mastoid (s) is worthy of note in regard to their general homology as cranial diapophyses. 

 The haemal or pleurapophysial arch of the third or frontal segment of the cranium is 

 modified to constitute the lower jaw. 



In the mandible of Ichthyosaurus the articular element (Tab. XXII, fig. 2, 29)^ is 

 scantly visible in an outside view, being covered by the largely developed and backwardly 

 extended surangular^ (ib. 30). With this the angular coextends or slightly surpasses 

 behind (ib., 31)-^ 



Of both elements the outwardly exposed surfaces (Tab. XX, fig. 2), as they advance, 

 gradually diminish to a point ; the surangular (30), in Ichthyosaurus covimunis, disappearing 

 between the dentary (33)* and splenial (32),^ in advance of the hinder half of the ramus; 

 the angular (31) terminates between the surangular (30) and splenial (32), as far behind 

 the fore half of the ramus. The hind part of the outer surface of the surangular shows a 

 triangular, almost flat, but feebly concave surface, finely sculptured with linear impres- 

 sions, converging forwards to the apex of the triangle, and indicative of a muscular inser- 

 tion. In advance of this part the surface is smooth and feebly convex, gaining in depth 

 by the convex curve of the lower border, and a similar one at the upper border, which 



' CuviER, ' Oss. Foss.,' V, pt. ii, p. 272, pi. xvi, figs. 4 and 5 {Faranits), d (' articulaire '). 



2 lb., ib., ' surangulaire,'y. 



^ lb., ib., 'angulaire,' e. 



* lb., ib., (Faranus), ' os dentaire,' a. 



^ lb., ib., ' operculaire,' b. 



