BRITISH FOSSIL REPTILES. 89 



feeble development of the cerebral cavity, and still more essen- 

 tially in the unanchylosed state of the composite cranial bones, 

 — a fact already referred to in the general characters of the 

 Enaliosauria. We shall see, moreover, that the connexions 

 of the bones partake more of the Lacertian than of the Croco- 

 dilian types ; but the Ichthyosaurus departs at once from both 

 the Cetacean and Saurian characters in the disproportionate de- 

 velopment of the intermaxillary, as compared with the max- 

 illary bones, and in the immense size of the orbits and the large 

 and numerous sclerotic plates : it is these modifications vrhich 

 give to the cranium of the Ichthyosaurus its peculiar features. 

 The occipital bone presents in a separate state the usual ele- 

 mentary pieces, called the basi- ex- and supra-occipital bones. 



In tracing the analogies of this composite bone, we first 

 observe that in the Crocodile the basi-occipital terminates be- 

 hind in a convex hemispheric tubercle, which articulates with 

 the wide adontoid appendage of the axis, and with the small 

 body of the atlas. Above the occipital tubercle there is a con- 

 cave surface on which rests the medulla oblongata. The ex- 

 occipitals articulate with the lateral boundaries of this surface, 

 and form no part of the tubercle for articulation v/ith the atlas. 

 In the Lacertians the ex-occipitals encroach considerably upon 

 the upper surface of the basi-occipital, diminishing the extent 

 which it affords for the support of the brain, and entering largely 

 into the formation of the articular tubercle by which the head is 

 joined to the vertebral column. 



In the Ichthyosaurus the ex-occipitals articulate to the whole 

 of the upper surface of the basi-occipital, which sends up a 

 compressed conical crest between them. The ex-occipitals also 

 form a portion of the vertebral articular tubercle, but in a less 

 degree than in most Lacertians. In this respect the Ichthyo- 

 saurus holds an intermediate position between the Crocodilian 

 and Lacertian Saurians. 



The restorations of the posterior region of the Ichthyosaurian 

 cranium hitherto given* are defective in regard to the relative 

 positions of the ex-occipitals to the basi-occipitals ; but the re- 

 presentation by Mr. Hawkins f approaches the closest to nature. 

 The under part of the basi-occipital expands and terminates 

 anteriorly in a pretty regular curve, with the convexity directed 

 forwards. There is in some species a slight emargination in the 

 middle of this curvature entering into the formation of the cir- 

 cumference of the Eustachian outlet of the basi-sphenoid. 

 The articidar tubercle of the basi-occipital frequently presents 



* Geol. Trans., 1822, p. 117. f Memoirs on Ichthyosauri, fol. PJ. I. 



