14 
FOSSIL REPTILIA OF THE 
side, and, in the dorsal region, d i — 60, are turned flat in that direetion. At the base 
of the tail, where these flattened surfaces again become diminished in extent, the 
vertebrae gradually resume their vertical or prone position, the summits of the spines 
being uppermost, as far as the seventieth (counting from the head), beyond 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 fln, 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 sid6 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 natural connections by the Lias stone, 
i. e., 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 iLomalospondylus 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 
dolichocleirus (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 thi’oughout the neck. Fig. 7, Tab. V, gives a view of the under surface of 
the eighth cervical vertebra ; fig. 6 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 w^ell exemplified in these, which may be profitably compared with the figures of 
the corresponding vertebrm of the Plesiosaurus jdanus, in a former Monograph,* as 
exemplifying the degree in which vertebral characters are developed in the different 
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 of the Palseontographical Society for 1862, issued in 1864. ‘Supplement No. II to the 
Monograph of the Fossil Reptilia of the Cretaceous Formations’ (Tab. I, figs. 20, 22, 25). 
