THE DIAPHRAGM OR MIDRIFF 
animals, it is not always possible to decide whether a 
given fragment is really ofa mammalorareptile. There 
is a famous fossil, the fragment of a skull of an animal, 
which has been named T7itylodon ; it is not fully agreed 
whether this creature was a mammal or a reptile. 
In the case of such a fragment as a bit of a long bone or 
a finger bone, the problem would be still harder, and 
often insoluble. 
With living mammals and living reptiles there is no 
difficulty at all in distinguishing them by bony and other 
characters. With a skull or the entire skeleton before 
him, no one could fail to recognize the few features to 
which attention has been directed. And there are 
of course many others, to enumerate which would be 
too long a task here. 
The internal anatomy of a mammalis constructed upon 
the same general plan as is that of a reptile or an am- 
phibian, as has been already set forth in our general 
description of the vertebrata. But there are important 
differences of minor weight which differentiate the 
mammal from allother vertebrates. The most important 
of these is the fact that in the mammal the heart and 
lungs lie in a separate chest cavity, divided off from the 
abdominal cavity, which contains the liver, intestines, 
kidneys, and so forth, by a muscular partition, known 
as the diaphragm. In all mammals this character 
holds good ; and in no animal which is not a mammal is 
there any structure exacily comparable to this dia- 
phragm. 
§ CLASSIFICATION OF MAMMALS 
Having now got at a notion of what a mammal is as 
compared with other vertebrates, it is requisite to see 
how far the mammalia can be conveniently subdivided 
among themselves ; for it will be plain to the least 
T3 
