CEYPTOCLEIDFS OXONIENSIS. 185 



The outer and posterior borders are both concave, thickening towards the massive 

 articular region. This bears two facets, that for the ischium being nearly semicircular, 

 and the diameter of the semicircle forming the line of division between it and the 

 acetabular surface. This latter makes an angle of about 145° with the ischial surface 

 and is slightly concave ; there is no contact between the pubis and ilium. 



The ischium (isc.) is of the usual hatchet-head form. The anterior portion of its 

 symphysial border is thickened and bears a deep symphysial surface (PI. X. fig. 3 b), 

 behind which it thins rapidly, thickening again a little towards its posterior angle. 

 The ischial symphysial surface was separated in front by a pad of cartilage, which, as 

 already mentioned, was probably continuous with that between the pubes ; probably 

 there was also a small posterior cartilage. The neck of the ischium is comparatively 

 narrow and depressed in section ; towards the articular surfaces the bone becomes 

 greatly thickened; there are three facets — one, looking almost directly forwards and 

 semicircular in outline, for the pubis, a median one rectangular in outline and slightly 

 concave forming the middle and greatest part of the acetabulum, and a posterior one 

 looking backwards and outwards but only a little upwards, for the ilium, which slopes 

 backwards much more than in Murcenosaurus ; the surfaces for the pubis and ilium 

 are roughened for cartilage, the acetabular surface is smooth. 



The ilium (il.) is a stout slightly curved rod of bone. Its lower end is greatly 

 thickened and bears a nearly flat oblique oval surface, the inner two-thirds of which 

 unite with the iliac facet of the ischium ; the remaining third, making a slight angle 

 with the rest, forms the posterior wall of the acetabulum. This surface was covered 

 with cartilage, which also extended up on to an angular projection marked c. in the 

 figure (PI. X. fig. 3 a). 



The middle part or shaft of the ilium is contracted and oval in section; it is curved, 

 the concavity being anterior, and on the middle of its posterior border there is a small 

 angular prominence probably for the attachment of muscles. The upper part of the 

 bone is compressed and somewhat like the blade of an oar ; the anterior border of this 

 expanded region is thin and sharp, the posterior thick and rounded. The inner face 

 of the upper end of the ilium is flat and, except in very old individuals, shows little or 

 no trace of any union with the sacral ribs, to which probably it was attached loosely by 

 ligaments. 



In the pelvis, as a whole, it will be noted that the ilium is greatly inclined backwards, 

 but this does not necessarily represent its exact inclination to the vertebral column, 

 because probably the whole pelvis was inclined downwards and forwards. The pubes 

 and ischia of opposite sides do not make a distinct angle with one another, their 

 median portions being almost on the same plane, and the visceral surface of the ventral 

 portion of the pelvis is only slightly concave from side to side. From before backwards 

 the line of the symphysis is convex on the visceral side, its highest point being only 

 about 8'5 cm. below a straight line joining the middle points of the acetabular cavities. 



2b 



