PELVIC GIRDLE BIRDS 



269 



extending backwards and downwards. Ornithischia (fig. 292) have a quadri- 

 radiate pelvis; the pubes do not meet in the middle line and lack a prepubis, 

 but have a ventral postpubis extending from beneath the acetabulum downwards 

 and backwards paralleling the long and slender ischium which bears an obturator 

 process. 



At first sight the Ornithischian pelvis resembles that of birds, and some would 

 recognize exact homologies between the two groups. Others hold, on the 

 grounds of ontogeny, that the avian pubis 

 is at first directed forwards and only later 

 comes to parallel the ischium (p. 270), giving 

 off, in the adult, a (pectineal) process which 

 simulates the pubis of the Ornithischian 

 Dinosaur. 



Pterosauria have an elongate pelvis 

 (fig. 293), the preacetabular part being the 

 longer. Ilium and ischium are fused, ex- 

 cluding the pubis from the acetabulum. 

 The pubis apparently was loosely attached 

 to the other bones, its distal end expanded 



in some species; while in some the pubes of the two sides met in the mid- 

 ventral line. 



AVES. — As Stated above, in spite of very marked superficial 

 similarities, the pelves of birds and Ornithischia are structurally very 

 different, the apparent similarities being doubtless to be explained 

 on the grounds of similarity of posture, these reptiles, hke the birds, 



Fig. 293. — Pelvis of Plerodactylus 

 (Zittel). il, ilium; is, ischium; p, 

 pubis. 



Fig. 294. — Development of pelvis of chick (Miss Johnson, '83). A, 6-day chick; 

 B, older; C, 20 days. Cartilage stippled; bone white, a, acetabulum; il, ilium; in, 

 ischiadic nerve; is, ischium; on, obturator nerve; p, pubis; pp, pectineal process. 



being largely bipedal. The early stages of the avian pelvis (fig. 294) 

 have parts resembhng those of embryo hzards, ischium and pubis 

 pointing towards the median plane, and having a deep incisure between 

 them in which is the obturator nerve, while the pubis has a marked 



