60 



MARINE REPTILES OF THE OXFORD CLAY. 



posterior border in an acute angle, the most posterior portion of the bone. Anteriorly 

 it passes by a broadly rounded angle into the anterior border. The inner face of the 

 expanded portion is somewhat roughened, but there is no clear evidence of contact with 

 the sacral ribs. In the other form (text-fig. 25, B) the upper border is nearly straight 

 and the anterior and posterior angles much more developed, particularly the anterior, 

 which instead of being rounded off is a prominent angle of rather less than 90°. Just 

 beneath it on the inner face of the anterior border there is a tuberosity which may 

 indicate union with the anterior sacral rib. In both forms beneath the expanded 

 blade the bone contracts into a shaft, roughly oval in section, the inner face being 

 flattened and the posterior rather sharper than the anterior border. At its ventral 

 end the bone is greatly thickened and bears two facets, one looking downwards, inwards, 



Two right ilia of Pelonemtes philarehus, showing the variation in form : A, ilium of R. 2441 ; 



B, ilium of R. 2438. (^ nat. size.) 

 acet., acetabular surface ; is.f., facet for ischium ; cr.L, crista ilii. 



and backwards for union with the ischium (is.f.), the other looking downwards and 

 forwards and forming the posterior portion of the acetabulum (acet.) ; both are 

 roughened and pitted as if for a covering of cartilage, and they make an angle of about 

 100° with one another. 



Hind Limb (text-fig. 23, B). — The hind paddle, as usual in the family, is larger than 

 the fore, the femur (fern.) especially being longer and relatively, perhaps, a little more 

 slender than the humerus. The head (h.) is rounded, and on its outer side joins the 

 trochanter, the upper face of which is flattened and rough, having been covered with 

 cartilage in life. The shaft is oval in section ; on the ventral face and posterior border 



