Page:Quarterly Journal of the Geological Society of London, vol. 26.djvu/112

26 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, Iguanodon, and Hypsilophodon, and, secondly, in that of the distal end of the tibia and of the astragalus, as evidenced by Poikilopleuron, Megalosaurus, and Lælaps.

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 acetabulum; and the acetabulum is either wholly closed by bone, or presents only a moderate-sized fontanelle, as in the Crocodilia.

In the Bird 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 remaining membranous. The anterior pier of the arch or præacetabular 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 character 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 extends 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 Rhea. The ischia never come together directly in a median ventral 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 Iguanodon it has the obturator 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 Iguanodon 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 Hypsilophodon, the ischia united in a median ventral symphysis in all the Dinosauria.