KIMMERIDGIAN PLIOSAURS. 153 



A cervical vertebra {Sauropterygia, PI. 18) of aPliosaiirus, fi^om the Eammeridge 

 Clay of Foxcombe Hill, near Oxford, measures, for example, in breadth six inches ; 

 in depth, or vertical diameter, five inches ; while in length, or the diameter corre- 

 sponding with the axis of the animal's body, or of its vertebral column, it measures 

 only two inches and a half. Nevertheless, with these ichthyosaurian proportions 

 is associated an essentially plesiosaurian type of structure. The lower surface 

 of the cervical centrum (ib., fig. 2) shows the pair of vascular foramina; the 

 terminal articular surfaces are flat or very sHghtly concave : the cervical 

 rib was ligamentously tied, in some species, to two processes, the di- and par- 

 apophyses (fig. 1), occupying two thirds of the fore-and-aft extent of the 

 side of the centrum, slightly projecting beyond the surface, and divided by a 

 deep linear fissure. I have rarely seen an instance in which the neurapophyses 

 were anchylosed to the centrum, and never one with the pleurapophyses so 

 attached. At the trunk end of the series the costal processes begin to climb, as 

 in Plesiosaurus, upon the neurapophyses, — the diapophysis growing at the expense 

 of the parapophysis, until the rib becomes supported, in the dorsal I'egion, upon a 

 single strong and prominent process : this is subdepressed, with an oval transverse 

 section, which is rather sharp at the anterior margin. The vertebral centrums begin 

 to gain in length as the costal processes rise in position, and those of the dorsal 

 region have attained to quite plesiosaurian proportions. Throughout the rest of 

 the column the vertebrje closely repeat the plesiosaurian characters on a large 

 scale. The sides, or non-articular surface of the centrum, are rugous near the 

 articular ends, elsewhere smooth, and in the dorsal region longitudinally concave. 

 In the caudal vertebras the costal process is undivided, prominent, with a vertically 

 elliptical section, continuous with the neurapophysial surface at the base of the 

 tail : the lower surface of the centrum is square-shaped and nearly flat : its angles 

 are marked by the hypapophysial surfaces, of which the anterior pair is usually the 

 largest. 



The generic character derived from the organs of locomotion is the apparent 

 absence of the antibrachial and cnemial bones, which seem to be represented by a 

 jDroximal row of three large " carpal " and " tarsal " ossicles. On the homology of 

 these I shall offer remarks in the sequel. 



As to the history of the present genus, I may briefly state that in a ' Pieport 

 on British Fossil Reptiles,' communicated to the Meeting of the British Association 

 for the Advancement of Sciences, held in 1839, and jarinted in the volume of 

 ' Reports ' for that year,^ I described certain fossils, from which were deduced the 

 two species of Plesiosaurus, called "• grmuUs" (p. 83), and " trochanterius" (p. 85). 

 In my second Report on the same class of fossils communicated to the Association 

 in 1841, I pointed out (p. 60) the characters by which these two species departed 



1 ' Eeport of Brit. Assoc.,' Svo, pp. S3, 86, 1839. 



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