THE GEOLOGICAL MIDDLE AGE. 
153 
been found ; so that no direct comparison can be 
made with the steps. It is, however, my belief, 
from all we know of the character of the Animal 
Kingdom in those days, that these animals were 
reptilian, but combined, like so many of the early 
types, characters of their own class with those of 
higher animals yet to come. It seems to me prob- 
able, that, in those tracks where one toe is turned 
backward, the impression is made not by a toe, 
but by a heel, or by a long sole projecting back- 
ward ; for it is not pointed, like those of the front 
toes, but is blunt. It is true that there is a divi- 
sion of joints in the toes, which seems in favor of 
the idea that they were those of Birds ; for when 
the three toes are turned forward, there are two 
joints on the inner one, three on the middle, and 
four on the outer one, as in Birds. But this fea- 
ture is not peculiar to Birds ; it is found in Tur- 
tles also. The correspondence of these footprints 
with each other leaves no doubt that they were all 
by one kind of animal ; for both the bipedal and 
the quadrupedal tracks have the same character. 
The only quadrupedal animals now known to us 
which walk on two legs are the Kangaroos. They 
raise themselves on their hind legs, using the 
front ones to bring their food to their mouth. 
They leap with the hind legs, sometimes bring- 
ing down their front feet to steady themselves 
after the spring, and making use also of their 
