﻿LIASSIC FORMATIONS. 



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lengthened, slender neck, and small head, in the capture of fishes or other active 

 marine prey. 



The whole framework of the trunk is singularly massive, and the character of this 

 part of the skeleton, as shown in the specimen (Tab. V), is especially striking in con- 

 trast with the slender neck and small head of the animal. 



Of the Skull (Tab. VI). 



The skull (Tab. VI), from the occiput to the end of the snout, is 9 inches long ; 

 it measures 4 inches 4 lines across the middle of the temporal depressions, 3 inches 

 6 lines across the occiput, which rises but 1 inch in height above the foramen 

 magnum ; the intertemporal part, or parieto-frontal crest, rises into a sharp ridge ; the 

 length of the temporal fossa is 2 inches 9 lines, the breadth is 2 inches. The diameter 

 of the orbit is 1 inch 6 lines ; from the fore-part of the orbit to that of the snout is 

 4 inches. The elliptical nostril shows a long diameter of about 6 lines, it is situated 

 about 8 lines in advance of the orbit, and about the same distance from its fellow. 

 The inter-narial portions of the nasal and premaxillary bones rise into an obtuse ridge. 

 The teeth are small, slender, slightly recurved at the fore-part of the jaw, where the 

 enamelled crown of the longest does not exceed 10 lines. No sutural evidence of 

 cranial structure is discernible ; the bones about and between the orbits show the 

 effects of pressure. Estimating the length of the skull by that of the lower jaw, about 

 two inches should be added to that taken from its exposed and visible part. 



This part of the skull (Tab. V) is susceptible of satisfactory comparison with the 

 corresponding region of the skull in the Plesiosaurus dolichodeirus (Tab. Ill, fig. 1) ? 

 the species which most resembles the Plesiosaurus homalospondgliis in the length of the 

 neck and the small proportional size of the head. 



By comparing Tab. Ill with Tab. VI, in which the skulls of the two species 

 are figured of the natural size, from probably mature individuals of average size, and 

 from the same aspect, the difference of proportion and form is such, and so obvious, 

 that, were two skulls of existing lizards to be so contrasted, it is probable that some 

 Erpetologists would be led to sever them more widely than by specific bounds. The 

 composition of the cranium, the position and relative size of its principal cavities, and 

 especially of the nostrils, the character of the dentition, are, however, so strictly 

 Plesiosaurian in the two fossil skulls here compared, that there is no sufficient ground 

 for encumbering the Sauropterygian group with one or two additional generic names. 



The skull of Plesiosaurus homalospondylus is longer in proportion to its breadth, 

 more oblong in shape, more obtusely terminated anteriorly. It is possible that the 

 skull of the Plesiosaurus dolichodeirus compared (Tab. Ill) may have suffered more 

 horizontal pressure, but not such as to have affected its triangular shape due to the 



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