14 BRITISH FOSSIL REPTILES. 



side, and, in the dorsal region, d i — co, are turned flat in that direction. At the base 

 of the tail, where these flattened surfaces again become diminished in extent, the 

 vertebrae gradually resume tlieir vertical or prone position, the summits of the spines 

 being uppermost, as far as the seventieth (counting from the head), be)fond which some 

 dozen of the terminal caudals are jumbled together in an irregular group, as if that 

 part of the carcass, supporting perhaps a caudal expanse of integument or fin, had been 

 subject to some disturbing influence prior to complete imbedding in the matrix. 



I conclude that this partial rotation of the dorsal series took place before the 

 petrifaction of the bones and bed ; because the ribs of the right side have slipped from 

 their attachments to the diapophyses, in a degree corresponding with the extent of the 

 rotation. For, had they been cemented in their natui*al connections by the Lias stone, 

 i. c, after the petrifaction of the mud, and prior to the operation of the extraneous 

 pressure, they might have been expected to have been bent or broken, when pressed 

 into the same plane with the neural spines, without any slipping from their previous 

 joints ; whereas this dislocation implies a rotting away of the articular ligaments, and a 

 certain yielding of the surrounding bed. 



The chief characteristics of the skeleton of the Plesiosaurus homalosjjondylus are, the 

 length of the neck, the height and breadth of the dorsal and contiguous cervical and 

 caudal spines, with the smallness of the head. The length of the neck is due 

 both to the number of vertebrae — thirty-eight, and to their proportionate length 

 individually, and chiefly to the latter character, as compared with Plesiosaurus 

 (Jolichodeirus (Tab. I). 



I caused to be carefully removed from the matrix of the present skeleton the 

 thirteenth and fourteenth (Tab. V, figs. 2 — 4) of these instructive vertebrae, the length 

 of the centrum in which agreed with that on which I had made notes and drawings in 

 1842. They corresponded in every other particular with these vertebrae. The low, 

 longitudinal ridge or rising (Tab. V, figs. 2, 5, r) on the side of the centrum may 

 be traced throughout the neck. Fig. 7, Tab. V, gives a view of the under surface of 

 the eighth cervical vertebra ; fig. G gives an end view, and fig. 5 a side view of the 

 centrum of the third cervical vertebra, all of the natural size. The specific characters 

 are well exemplified in these, which may be profitably compared with the figures of 

 the corresponding vertebrae of the Plesiosaurus planus, in a former Volume,* as 

 exemplifying the degree in which vertebral characters are developed in the diS"erent 

 species of the genus. 



The cervical ribs, as indicated by the articular surface (Tab. V, figs. 2, 7, pi), are of 

 small size in proportion to the rest of the vertebra, until about the thirtieth, in which 

 the transverse outstanding part of the stem is two inches three lines in length, and the 

 longitudinal part two inches six lines. In the thirty-fourth vertebra this has attained 



* Volume I, ' British Fossil Reptiles ;' Supplement No. II to the ' Fossil Reptilia of the Cretaceous 

 Formations.' 



