MUE.ilNOSAUEUS. 117 



forms the anterior border of the obturator foramen {oM.f.). Between the posterior and 

 lateral borders is the thickened and massive postero-external portion of the bone, which 

 bears the acetabular and ischial surfaces. The former of these is much the larger : its 

 form is that of half an elongated oval, the short diameter of which is the line of union 

 with the ischial surface ; it is gently concave in a longitudinal direction and forms the 

 greater part of the surface for the articulation of the femiir ; there is no union with the 

 ilium. The ischial surface is roughly triangular in outline, the angles being rounded ; 

 it makes an angle of about 145 degrees with the acetabular surface. Both of these 

 surfaces are roughened and were cartilage-covered, but on the acetabular surface the 

 roughness is slight. 



The ischium (Pis. IV. & V. fig. 9 ; isc., text-fig. 6-5) is shaped somewhat like the 

 Head of a hatchet, consisting of a massive articular portion united by a comparatively 

 narrow neck with the broad ventral blade. The head bears three articular surfaces. 

 The anterior one {pu.f.) looks directly forwards ; it is roughly triangular and unites 

 with the corresponding surface of the pubis. Behind this and making an angle of 

 about 90° with it, is the acetabular surface (acet.), roughly quadrate in outline and 

 looking directly outwards. Behind this again, and making a very obtuse angle with it, 

 is the triangular surface for the ilium (il-f-), looking upwards and backwards. The 

 acetabular surface is comparatively smooth, but the others are raised into strong ridges 

 with deep pits between them. Beneath the massive head the bone is much narrowed, 

 and at the same time is compressed from above downwards, passing into the broad 

 ventral expansion. The anterior and posterior borders of the bone are both concave, the 

 anterior more strongly so ; the anterior is rounded, while the posterior is sharp in the 

 region of the neck of the bone, but towards the posterior angle it becomes roughened 

 for the attachment of muscles. The median border in its anterior (symphysial) part is 

 nearly straight, and, owing to the thickening of the bone, the symphysial surface (s^m.) 

 is deep, narrowing both forwards and backwards. Behind the symphysis the inner 

 border of the ischium curves sharply outwards, meeting the posterior border at an 

 angle of about 90° ; this portion of the inner border has a grooved edge, and was 

 fringed with cartilage during life. The ventral surface of the blade of the bone 

 is nearly flat, except where the symphysial thickening occurs ; the upper surface is 

 gently concave. The symphysis of the ischia is not continuous with that of the pubis, 

 so that so far as the bones are concerned the obturator foramina of opposite sides are 

 in free communication with one another, but probably in life they were separated by a 

 median band of cartilage. 



Hind Limb (PL IV. fig. 10; PL V. fig. 12; text-fig. 63, A).— The femicr is in 

 most respects very similar in structure to the humerus, the most notable difference 

 being the smaller extent to which the distal end is expanded. The head (h.) seems 

 never to become completely ossified, but even in old individuals is only moderately 



