224 THE ANATOMY OF VERTEBRATED ANIMALS. 



This transitional character of the Ornithoscelidan 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 Grocodilia. 



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

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

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

 branous. The anterior pier of the arch, or pree-acetabular pro- 

 cess, extends farther downward than the posterior pier, or 

 post-acetabular process. 



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

 front of the acetabulum, and furnishes only a widely-arched roof 

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

 the further proportional extension of the post-acetabular pro- 

 cess downward (Fig. 78, B.). 



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

 elongated bone, which becomes connected with the pubis in 

 the acetabulum, and extends downward, inward, and somewhat 

 backward, to unite with its fellow in a median ventral sym-- 

 physis. The obturator space is not interrupted by any for- 

 ward process of the outer and anterior half of the ischium. 



In all birds (Fig. 78, A.), the ischium is elongated and in- 

 clined backward, the backward direction being least marked 

 in Apteryx^ and most in Rhea. The ischia never come to- 

 gether directly in a median ventral symphysis, though they 

 unite dorsally in Rhea. The anterior edge of the external, or 

 acetabular, half of the ischium very generally sends off a pro- 

 cess which unites with the pubis, thus dividing the obturator 

 space. 



In all the Omithoscelida (Fig. 78, B.), in which I have 

 been able to identify the bone {Thecodontosaurus, Terato- 

 smirus, Megalosaurus, Iguanodon, Stenopelyx, Madrosaurus, 

 Ilypsilophodon), the ischium is greatly elongated. In Iguaii' 

 odon it has the obturator process characteristic of the same 

 bone in Birds ; and I imagine that the same process is seen in 

 Gompsognathus. In Sypsilophodon there can be no mistake 

 about the matter, and the remarkable slenderness and prolon- 

 gation of the ischium give it a wonderfully ornithic character. 



