LIASSIC FORMATIONS. 97 



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

 extended surangular 1 (ib. 30). With this the angular coextends or slightly surpasses 

 behind (ib., 31). 2 



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

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

 between the dentary (33) 3 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 

 feebly simulates a coronoid process. Opposite, or beneath the slender lower boundary of 

 the orbit, the surangular is overlapped by the hinder-jointed end of the dentary (33). An 

 oblique canal opens forward below the coronoid rising, beneath the hind border of the 

 orbit, whence extends forward a channel, becoming shallow and ending beneath the over- 

 lapping point of the dentary. The angular (31) gradually diminishes in breadth or 

 depth as it extends forward. 



From such views as I have been able to obtain of the inner surface of the mandibular 

 ramus, the articular element (PI. XXVI, fig. 2, 29), after developing the concavo-convex 

 broad surface for the tympanic, seems to be continued forward as a thinner, deeper 

 plate, corresponding to the ' complementaire ' of Cuvier. 5 It, however, develops no 

 coronoid process in Ichthyopterygians, but in some specimens seems to be a detached 

 scale (PL XXVI, figs. 2 and 3, 30'), simply applied to the inner surface of the ramus over 

 the line of junction of the surangular and angular elements touching or joining anteriorly 

 the hind end of the splenial. 



This element (PL XXIV, fig. 2, 32 ; PL XXVI, figs. 3— G, 32) begins behind by a point 

 between the surangular and angular, gains breadth as it advances to become applied to 

 the inner side of the dentary, which it also underlaps and strengthens usually to within 

 the anterior fourth part of the length of that element. The splenial in Ich. communis 

 contributes a small part of the outer side of the jaw beneath the anterior pointed end of 

 the surangular. 



There is no other indication of lack of outer surface of the ramus than the neuro- 

 vascular foramen and groove of the surangular and smaller irregular foramina in the 

 dentary. 



1 Cuvier, « Oss. Foss., v, pt. ii, p. 272, pi. xvi, figs. 4 and 5 ' surangulaire,'/ 



2 lb., ib., ' angulaire,' e. 



3 lb., ib., (Varanits), 'os dentaire,' a. 



4 lb., ib., ' operculaire,' b. 



5 Loc. cit., p. 272, c. 



