26 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. [Nov. 10, 
what I say about the fibula is consistent only with a knowledge of 
the proper relations of its ends. 
The further evidence as to the ornithic affinities of the Dinosauria 
which I have to bring forward in the present paper consists, first, in 
the structure of the pelvis, as shown by Megalosaurus, Iqguanodon, 
and Hypsilophodon, and, secondly, in that of the distal end of the 
tibia and of the astragalus, as evidenced by Potkilopleuron, Megalo- 
saurus, and Lelaps. 
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 the ilium is not prolonged in front of the aceta- 
bulum ; and the acetabulum is either wholly closed by bone, or pre- 
sents only a moderate-sized fontanelle, as in the Crocodiha. 
In the Bird the ilium is greatly prolonged in front of the aceta- 
bulum, and the roof of the acetabular cavity is a wide arch, the inner 
wall of that cavity remaining membranous. The anterior pier of 
the arch or preeacetabular process extends further downwards than 
the posterior pier or postacetabular process. 
Now, in all the Dinosauria which I have yet examined, 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 cha- 
racter in the further proportional extension of the postacetabular 
process downwards. 
2. The ischium in the Reptile is a moderately elongated bone, 
which becomes connected with the pubis in the acetabulum, and ex- 
tends downwards, inwards, and somewhat backwards, to unite with 
its fellow in a median ventral symphysis. The obturator space is not 
interrupted by any forward process of the outer and anterior half of 
the ischium. 
In all birds the ischium is elongated and inclined backwards, the 
backward direction being least marked in Apteryx, and most in 
fihea. The ischia never come together directly in.a median ven- 
tral symphysis, though they unite dorsally in Rhea. The anterior 
edge of the external half of the ischium very generally sends off a 
process which unites with the pubis, thus dividing the obturator 
space. 
In all the Dinosauria in which I have been able to identify the 
bone (Thecodontosaurus, Teratosaurus, Megalosaurus, Iguanodon, 
Stenopelyx, Hadrosaurus, Hypsilophodon), the ischium is greatly 
elongated. In Jguanodon it has the obtwrator process characteristic 
of the same bone in Birds ; and I imagine that the same process is 
seen in Compsognathus. In Hypsilophodon there can be no mistake 
about the matter, and the remarkable slenderness and prolongation 
of the ischium gives it a wonderfully ornithic character. In Jgua- 
nodon this slenderness and prolongation are carried beyond what is 
to be seen in Birds. I am disposed to think, however, that, as 
was certainly the case in Hypsildphodon, the ischia united in a 
median ventral symphysis in all the Dinosauria. 
