98 FOSSIL REPTILIA OF THE 



This important element (PL XXIV, fig. 2, 33 ; PI. XXVI, 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 (PL XXVI, figs. 4 and 5, 32). As 

 the dentary advances it supplants the splenial by developing an inner wall, 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. 1 



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. 

 The longitudinal central vacuity of the mandible or interspace between its several con- 

 stituents is considerable, as is shown in Clift's figure of a transverse section of a 

 ramus. 3 



After comparison of the foregoing structure with the homologous bone in Meptilia, I 

 may remark that the mandible of Ichthyosaurus differs from that in Lacertilia in the 

 minor relative size and backward extension of the ' articular ' (29) ; 3 it resembles more 

 the Crocodilia in the major relative size of the angular (31), but exceeds in its proportion 

 and position as forming the angle of the jaw, though it is less produced backwards. It 

 differs from both the Crocodilian and Lacertian jaws in the larger relative size of 

 the surangular, which extends backwards so as almost to conceal the articular from 

 outward view. It resembles the Crocodilia in the absence of any coronoid process 

 from a complementary element ; also in the larger relative size and length of the 

 ' splenial ;' in this character the Monitors depart less than do the Iguanas from the 

 Ichthyosaurs. In the dentary element Ichthyosaurus shows the important affinity to 

 lacertilia in the dental groove, devoid of alveolar partitions ; but in the rudimentary 

 indication of these there is a resemblance, as before remarked, to the short non-alveolar 

 tract at the back of the dental series in some Crocodilia. The mandible of Ichthyosaurus 

 notably differs from that of Crocodiles, and resembles that of Lizards, in the absence of 

 the vacuity between the angular, surangular, and coronal elements, and in the presence 

 of the longitudinal series of neurovascular foramina or outlets along the outer side of the 

 dentary element. 



In the greater extent of the mandibular symphysis Ichthyosaurus differs from both 



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

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



* ' Philos. Trans,' 1820, pi. xvi. See also PI. XXVI, figs. 4 and 5, of the present Monograph. 

 3 Cuyieb, torn, cit., pi. xvi, figs. 4, 8, 13, d. 



