THE DINOSAURIA. 
57 
outward to the proximal end of the bone. Both the trochanters 
are sometimes wanting in American genera. In Belodon, the 
femur diverged hut little from the shape in living Crocodiles, 
hut its slight modifications are all in the Dinosaurian direction ; 
and it appears to me, that, like so many other of the elements 
of the skeleton, the femur underwent evolution in successive 
genera, which caused its original somewhat Crocodilian appear- 
ance afterwards to approximate to the shape that characterizes 
the hone in Birds and Mammals. 
The next segment of the hind-limb, consisting of the tibia 
and fibula, is sometimes much more Avian in its character, but 
differs fundamentally from this part of the bird’s skeleton in 
having the fibula better developed, so that it extends down to 
articulate with the tarsus, while in birds it tapers away to a 
needle point, and never is inserted into the tarsus at all. The 
i\.vian character is, therefore, to be found in the tibia only. 
And here it is limited to two points. First, the development 
of a strong anterior crest, which is directed forward and out- 
ward, so as often to extend in front of the fibula, is a Dino- 
saurian character met with, to some extent, among Birds, but 
quite as well marked in Mammals. Secondly, the shape of the 
distal end of the bone is bird-like ; but that form is found in 
no adult bird, and is only to be detected in the young bird 
before the tarsus has become blended with it ; so that while it 
might go to show, perhaps, that Birds are descended from a 
common stock with Dinosaurs, it would be misleading to regard 
it as altogether Avian, since the character is lost in the adult 
bird’s skeleton. It is only in such genera as Coelosaurus 
(Fig. 16), Megalosaurus, and Iguanodon, that the tibia has a 
shape that suggests the bird ; while in Hylaeosaurus (Fig. 15), 
it is, if not Crocodilian, more suggestive of a crocodile than 
a bird ; and, so far as can be inferred from figures of the 
Colorado type named Morosaurus (Fig. 10), the tibia in that 
genus is altogether Reptilian. But when the tarsus, or rather 
the astragalus, is closely applied to the tibia, as in Megalo- 
saurus, Pcecilopleuron, Laelaps, or Iguanodon, it gives the 
bone a resemblance to the bird which is almost convincing, 
since the parallel extends to nearly every detail. The character, 
however, is shorn of much of its importance, when we remember 
that there are many Dinosaurs in which there is an os calcis, 
or heel-bone, placed side by side with the astragalus (Fig. 17) ; 
and that in other genera like Morosaurus, the astragalus was 
not an Avian conformation, so that it is not a general charac- 
teristic of the order, and, therefore, must have undergone 
change in the group. It seems, hence, more reasonable to 
infer that the Avian type of the bone is more likely to be 
a consequence of the hind-limbs being used for progression 
