THE OENITHOSCELIDA. 261 



have been so much raised. The Crocodile's limb is purposely repre- 

 sented in an unnatural position. In nature, the lemur would be 

 turned out nearly at right angles to the middle vertical plane of the 

 body, and the metatarsus would be horizontal. The letters are the 

 same throughout. //, ilium ; Is, ischium ; Pb, pubis ; a, anterior pro- 

 cess, 6, posterior process, of the ilium ; Tr, inner trochanter of the 

 femur ; T. tibia; F., fibula ; As, astragalus ; Ca, calcaneum. /., //., 

 III., IV., the digits. 



Remains of these Reptiles have hitherto been found only 

 in strata, which probably belong to the Triassic formation, 

 in India and South Africa, and the Ural mountains. 



VIII. The Oenithoscelida. — The very remarkable ex- 

 tinct reptiles vs^hich constitute this group, present a large 

 series of modifications intermediate in structm-e between 

 existing Reptilia and Aves. 



This transitional character of the Omithoscelidan skeleton 

 is most marked in the pelvis and hind limbs. 



If the pelvis of any existing reptile be compared with 

 that of any existing bird, the following points of difference 

 will be observed : — 



1. In the reptile (Fig. 78, C), the ilium is not prolonged 

 in front of the acetabulum ; and the acetabulum is either 

 wholly closed by bone, or presents only a moderate-sized 

 fontanelle, as in the Crocodilia. 



In the bird (Fig. 78, A.), the ilium is greatly prolonged 

 in front of the acetabulum, and the roof of the acetabular 

 cavity is a wide arch, the inner wall of that cavity re- 

 maining membranous. The anterior pier of the arch, or 

 prse-acetabular process, extends further downwards than 

 the posterior pier, or post-acetabular process. 



But, in all the Ornithoscelida, the ilium extends far in 

 front of the acetabulum, and furnishes only a vsidely arched 

 roof to that cavity, as in birds. It retains a reptilian 

 character in the further proportional extension of the post- 

 acetabular process downwards (Fig. 78, B.). 



2. The ischium, in the reptile (Fig. 78, C.) is a moderately 

 elongated bone, which becomes connected with the pubis in 

 the acetabulum, and extends downwards, inwards, and some- 



