56 BRITISH FOSSIL REPTILES. 



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 

 Tamus.^ 



After comparison of the foregoing structure with the homologous bone in Beptilia, 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) f it resembles more 

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

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

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

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

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

 from a coniplementary 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 Ichthyosaunis 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 

 Crocodiles and Lizards ; in these, it is limited to a small part of the dentary element, 

 but in Ichthyosaurus it includes both dentaries and splenials, and the ' symphysis dentarii ' 

 (Tab. XXI, fig. 2, 33') is to be noted together with the ' symphysis splenii ' (ib. ib., 32') in 

 the discrimination of species. Thus, the latter is longer than the dentary symphysis in Ich. 

 intermedius, but is shorter in Ich. conmmnis, and, relatively, much shorter in Ich. tenui- 

 rostris (ib., fig. 3), Ich. longirostris (ib., fig. 2), and other long and slender-jawed kinds. 



The vomerine bones are narrow, longitudinal, vertically disposed plates, with their 

 lower margins rarely visible at the interspace of the palatines and pterygoids. 



The prefrontal (Tab. XX, fig. 1, 14) I, as yet, know only by its external or facial 

 part. This is a narrow, moderately long, bony tract, extending from the postfrontal to 

 near the nostril (»*), there receiving the upper angle of the lacrymal (73) in a notch, the 

 upper branch of which notch is wedged between the lacrymal and nasal, (1.5) ; the lower 

 boundary is bent inwards to form part of that of the orbit. The extensive upper and 

 inner border of the prefrontal articulates with the nasal, the short hinder border with the 

 postfrontal. 



The position and relations of the prefrontal in Ichthyosaurus, as in some Fishes, 



1 ' rhiloB. Trans,' 1820, pi. xvi. See also PI. XXII, figs. 4 and 5, of the present Chapter. 



2 CuviEB, torn, cit., pi. xvi, figs. 4, 8, 13, d. 



