Page:Quarterly Journal of the Geological Society of London, vol. 26.djvu/96

10 and is firmly united with the pubis, and a posterior process which becomes very thick behind and articulated with the postacetabular process of the ilium. The shaft of the bone is flattened laterally, and has a thick and rounded posterior edge. Anteriorly it is thinner, and at 2⋅75 inch from the acetabulum it is produced into a broad and long decurrent process, the free edge of which overlaps the pubis. Such a process is very generally developed in birds. Beyond this process the ischium widens out, and seems to terminate in a spatulate free end, like that of the corresponding part of the ischium in Megalosaurus and Iguanodon. This spatulate extremity is broken away from the right ischium, but remains on the other side (Is'). And their relative position leads to the belief that the two bones united in a ventral symphysis. The long diameters of the ischia and pubes are parallel, and they are directed downwards and backwards in such a manner as to make an obtuse angle with the anterior half of the long diameter of the ilium. Hypsilophodon is the first reptile in which this disposition of the ischium and pubis has been observed.

Eleven caudal vertebræ in series, with a rudiment of a twelfth at the posterior end, and another which lies on one side of these, all belonging to the anterior part of the tail, are represented in the plate cited, but they have been worked out much further since it was published. The characters of some of the vertebræ are very well shown. For example, the third from the anterior end in the series of eleven now exposed, is 2 inches high from the lower edge of the centrum to the summit of the spine (Pl. I. figs. 6 & 7). The centrum is 0⋅8 inch long, while its articular faces are 0⋅6 inch high. The transverse measurements of the articular faces of the centrum cannot be ascertained in this vertebra; but that of the posterior face of the vertebra which lies by itself is 0⋅5, the length of the vertebra being 0⋅7 inch. The spine of the "third" vertebra measures, from the postzygapophysis to its truncated extremity, 0⋅85 inch, and 0⋅36 inch from before backwards; the spines of all the caudal vertebræ are slightly inclined backwards. The root of a transverse process (or caudal rib), 0⋅36 inch long, stands out at right angles from the upper part of the side of the centrum, its posterior edge inclining forwards. The under face of the centrum is concave from before backwards, and presents a narrow and flattened surface, traversed by a longitudinal groove. The zygapophyses are long, and the planes of their articular faces are almost vertical. The obliquely truncated surfaces for the articulation of the chevron bones at the anterior and posterior ends of the ventral face of each centrum are well marked. No chevron bone is attached to the vertebra under consideration; but several lie, one on the top of the other, beneath the fifth to the eighth vertebra of the caudal series. The best-preserved of these is 1⋅75 inch long, 0⋅38 inch wide at the vertebral end (Pl. I. fig. 8). The vertebral ends of the forks of the chevron bones are expanded and ankylosed together in the manner characteristic of the Dinosauria.

The length of the left femur is 5⋅7 inches, or rather less than the