TMETBIOKHYNCHUS. 169 



its outer face is convex or flat from above downwards, while the inner is concave and 

 fitted closely against the outer face of the rib of the axis (text-figs. 61, A, and 67, A). 

 Although in some specimens there is a clear trace of a diapophysial prominence at 

 the base of the neural arch of the atlas (see Hulke, Proc. Zool. Soc. 1888, pi. xviii. 

 fig. 1), no atlantal rib has been found in which there is any trace of the bifurcation of 

 the proximal end, though perhaps there may have been a ligamentous connexion with 

 the diapophysis, as there probably was in the case of the rib of the axis of Steneosaurus 

 (see above, p. 103, text-fig. 39, B). 



The rib of the axis (text-fig. 67, A) is deeply bifurcate, the head articulating with a 

 surface borne partly on the odontoid and partly on the ventro-lateral border of the 

 centrum of the axis (text-fig. 61, A) ; the upper (tubercular) branch articulates with 

 the well-developed diapophysial process borne on the base of the arch of the axis. 

 The capitular branch and the body of the rib are in the same straight line, and are 

 concealed beneath the atlantal rib, when the bones are in their natural position 

 (text-fig. 61, A) ; the outer face is gently convex from above downwards, the inner 

 concave. At the posterior end of the inner face there is a facet, the function of 

 which is not clear. The cervical ribs (text-fig. 67, C) behind the axis are very 

 similar to those of Steneosaurus in general structure, but differ in having a pro- 

 portionately longer and more compressed capitular (parapophysial) process, while the 

 tubercular (diapophysial) process is stouter ; this at least is the case with Metrio- 

 rhi/nchus super ciliosum. In M. Iceve the cervical ribs are more slightly constructed. 

 In the posterior cervicals and anterior dorsals the anterior limb of the body becomes 

 reduced to a crest, while the posterior portion elongates to form the main part of the 

 rib, which is grooved posteriorly. In this region (text-fig. 67, D, E) the capitular 

 and tubercular branches form a distinct angle with one another, but further back in 

 the dorsal series the tubercular (diapophysial) articulation merely forms a step-like 

 prominence on the posterior face (text-fig. 67, F) of the upper end of the rib, the 

 posterior border of which, between the capitular and tubercular facets, is closely 

 applied to the anterior edge of the transverse process, the line of contact being 

 marked by a rugose surface. The capitular and tubercular facets do not seem to 

 have approached one another towards the hinder end of the series as in the modern 

 Crocodiles and, to some extent, in Steneosaurus, but remain widely separated to 

 the last. In the dorsal region the bodies of the ribs are oval to circular in 

 section, and there is a slight ridge on the upper part of the posterior surface running 

 down from the tubercular facet. The sacral ribs have already been referred to 

 (text-figs. 63, 64). 



The caudal ribs (transverse processes) are borne on the first thirteen or fourteen 

 vertebrae; they are strongly compressed from above downwards, and decrease rapidly 

 in size from before backwards; they seem to be borne mainly or entirely on the 

 centrum, at a lower level than in Steneosaurus. 



PART II. z 



