The Skeleton 85 
a dorso-laterally projecting crest; medially it is 
firmly united to the sacral ribs (Fig. 18, 5) while its 
outer side forms the upper and greater part of the 
Fic. 28. PELVIS AND SACRUM OF AN ALLIGATOR (Caiman 
latirostris). X%. (Brit. Mus.) (After Reynolds.) 
1. ilium. 6. neural spines of sacral verte-; 
2. ischium. brae. ; 
3. true pubis. 7. symphysis ischii., 
4, epipubis (so-called pubis). 8. process bearing prezygapo-) 
5. acetabular foramen. physis. , 
acetabulum. Its outer and lower border has two 
surfaces, the larger and more posterior articulating 
with the ischium, the other with the cartilaginous 
pubis. 
The ischium (2) is a slightly arched bone, its 
ventral end a flattened blade articulating with its 
fellow, its dorsal end enlarged and thickened to 
articulate with the ilium, pubis, and epipubis. 
