490 O. C. Marsh—On Dinosaurian Leptiles. 
Just what this posterior extension of the calcaneum signi- 
fies in this case, it is difficult to decide on the evidence now 
known. It may be merely an adaptive character, as allopus 
appears in nearly every other respect to be a true carnivorous 
Dinosaur. It may, however, be an inheritance from a Croco- 
dilian ancestry, and preserved by a peculiar mode of life. 
Whatever its origin may have been, it was certainly, during 
the life of the animal, an essential part of the remarkable 
leaping foot to which it belonged, and in which it has since 
kept its position undisturbed. The presence of such an 
element in the foot of this diminutive Dinosaur certainly 
suggests that the group Hallopoda, which I have considered 
an order, stands somewhat apart from the typical Zheropoda, 
but not far enough away to be excluded from the subclass 
Dinosauria, as I have defined it in the present communication. 
In the genus Zanclodon, which is from essentially the same 
geological horizon in Germany as Aétosaurus and Belodon, 
we have one of the oldest true Dinosaurs known, and a typical 
member of the order Zheropoda. In the pelvic arch of this 
reptile, the ilium and ischium are in type quite characteristic 
of the group to which it belongs, but the pubic elements are 
unique. They consist of a pair of broad, thin plates united 
together so as to form an apron-like shield in front, quite 
unlike anything known in other Dinosaurs. The wide pubic 
bones of Selodon, and the corresponding plates in some of the 
Sauropoda (Morosaurus, figure 8), indicate that this feature of 
the reptilian pelvis may have been derived from some common 
ancestor of a generalized primitive type. The known trans- 
formations of this same pelvic element in one other order of 
Dinosaurs (the Predentata) make the modifications here sug- 
gested well within the limits of probability. The hind limb 
of one genus of this order is shown in figure 6. 
The skulls of Aétosaurus and Belodon both show features 
characteristic of some of the Dinosaurs, especially of the 
— Sauropoda, but these features need not be discussed here. 
The relation of Dinosaurs to Birds, a subject of importance, 
must also be postponed for another occasion. One point, 
however, may be mentioned in this connection. The pelvic 
bones of all known Birds, living and extinct, except the genus 
Archeopteryx, are coossified, while in all the known Dinosaurs 
they are separate, excepting Ceratosaurus (figure 9) and 
Ornithomimus. Again, all known adult Birds, living and 
extinct, with possibly the single exception of Archwopteryz, 
have the tarsal bones firmly united (figure 11), while all the 
Dinosauria, except Ceratosaurus (figure 10), have these bones 
separate. The exception in each case brings the two classes 
near together at this point, and their close affinity is thus 
rendered more probable. 
