MUE^NOSAUEUS. 105 



and the anterior one is usually the larger ; as these bones are followed towards the end 

 of the tail they become shorter and shorter (text-fig. 59, H, G, F), disappearing entirely 

 in the last few vertebrae. 



The cervical ribs (r., PI. IV. figs. 2, 3, 4 ; VI. VI. figs. 4, 5) are all single-headed, 

 the only sign that they may at one time have possessed two heads being perhaps 

 shown in the peculiar form of the rib-facets in some of the cervical vertebrae of 

 Murcenosaurus durohrivensis, where the articular surface is produced a little upwards 

 into a sort of small dorsal lobe (text-fig. 50, A, r.f.), which, hov/ever, is quite continuous 

 with the remainder of the surface. 



On the atlas the rib (r.^, text-fig. 49) is merely a small pointed process, but on the 

 axis {r.^) it is much larger and similar to the succeeding cervical ribs. These vary 

 in form considerably in the different species. In Murcenosaurus leedsi (PL IV. figs. 2-4) 

 they are strongly compressed from above downwards, and on the anterior portion of 

 the neck have a well-marked anterior angulation of the blade, thus showing some 

 approximation to the hatchet-like form found in many Liassic Plesiosaurs and in 

 Picrocleidus belocUs. In M. platyclis the ribs are likewise greatly compressed from 

 above downwards into thin plates ; but the anterior angulation is well marked only in 

 the anterior vertebrte in some specimens, and at the back of the neck they become 

 broad, thin, and slightly backwardly directed plates of bone (PL VI. fig. 4). In both 

 these species there is a longitudinal ridge on the side of the centrum immediately above 

 the facet for the rib (PI. IV. figs. 2, 3). In Murcenosaurus durohrivensis the cervical 

 ribs are much less compressed than in the other species, especially in the posterior part 

 of the neck, where they are oval in section, at least near the base. 



The dorsal ribs (text-fig. 60, C, D) articulate with the ends of the transverse 

 processes by large slightly concave oval surfaces, the long diameter of whicli is 

 vertical. Towards the articulation the bone becomes somewhat compressed from before 

 backwards, and its surface bears strong ridges for the attachment of muscles. Further 

 out it becomes rounded in section and terminates abruptly at its ventral end in a 

 concave surface, indicating that in life it was tipped with cartilage. 



There were either three or four pairs of sacral ribs (text-fig. 56, s.r.), which differ 

 from one another considerably in form, and their arrangement with regard to one 

 another is not certain. They are all stout bars of bone, enlarged at their extremities, 

 the inner end of each bearing a strongly convex surface for union with its centrum, witli 

 which it may fuse ; the upper border of the proximal end may also bear a facet («'./•) 

 looking upwards and articulating with the outer part of the pedicle of the neural 

 arch. The outer end terminates in a convex facet {s.f.), which no doubt was connected 

 in some way with the ilium. In some of the sacral ribs the shaft is curved, so that 

 probably they converged towards their outer ends. 



The caudal ribs (text-figs. 58, 59, cr.) articulate with the centra by strongly convex 

 facets ; they are compressed from above downwards, sometimes to a high degree, and in 



p 



