LIASSIC ICHTHYOSAURS. 55 



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 (3l) gradually diminishes in breadth or 

 depth as it extends forwiird. 



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

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

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

 plate, corresponding to the ' complenientaire ' of Cuvier.^ It, however, develops no 

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

 scale (ib., 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 (Tab. XX, fig. 2, 32 ; Tab. XXII, figs. 3 — 6, 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 M^ithin 

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

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

 the surangular. 



There are no other indications of lack of outer sui-face of the ramus than the neuro- 

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

 dentary. 



This important element (Tab. XX, fig. 2, 33 ; Tab. XXII, figs. 4—7, 33) is the longest, 

 if not largest, constituent of the composite mandibular ramus ; it seems not to have carried 

 in any species its symphysial articulation with its fellow to actual confluence. It affords 

 for the lodgment of the mandibular teeth at the hinder half or more of that series only 

 the outer wall and more or less of the floor of a broad and shallow alveolar channel, the 

 inner wall being here supplied by the splenial element (Tab. XXII, figs. 4 and 5, 32). As 

 the dentary advances it supplants the splenial by developing an inner Avail, which finally 

 rises so as to exceed in height the outer one (ib., figs. 6 and 7, 33). The inner surface of 

 the outer wall of the dentigerous groove shows feeble vertical ridges, indicative of alveolar 

 compartments, like those seen along the hinder terminal part of the same channel in 

 Crocodilia.^ 



On the outer surface of the dentary, a little below the alveolar border, a series of vas- 

 cular foramina and grooves leading thence forward is seen in most species of Ichthyo- 

 saurs ; in a few species the same surface is indented by a narrow longitudinal furrow. 



^ Loc. cit., p. 272, c. 



^ ' Descriptive Catalogue of the Osteological Series contained in the Museum of the Royal College of 

 Surgeons of England,' 4 to, 1853, vol. i, p. 167, Specimen No. 765. 



