EGGS AND EGG LAYING 
the Crocodilia. It is quite conceivable, and indeed one 
cannot help arriving at the conclusion, that mammals 
recognize each other far more by odour than by sight. 
Perhaps this physiological fact has something to do 
with the general absence of brilliant hues among the 
mammalia. There is no difficulty therefore about 
distinguishing a mammal from any other vertebrate 
by merely inspecting it in a cage. But it will be 
necessary to delve a little further into the subject, and 
to add something to the characters of this group from 
a consideration of certain anatomical features. The 
reason for this is obvious. For without reliable dis- 
tinguishing marks derived from the skeleton it would be 
impossible to place accurately the bones and teeth of 
fossil forms, of whose soft parts, including the skin, no 
trace remains in the rocks. It is true that we are not 
specially concerned here with extinct mammals or 
fossil vertebrates generally. But it would be taking 
a too limited view of zoology to completely ignore the 
fact that there were, as well as ave, mammals. A very 
marked feature of the mammalian skeleton is the fact 
that the bones of the vertebral column as well as for 
the limbs are not, so to speak, in one piece. At either 
end of the vertebra, for example, is a little disc known 
as the epiphysis, which ossifies separately from the main 
body of the vertebra, and remains distinct for a long 
period. The same takes place at the ends of the long 
bones of the arm and leg and the smaller bones of those 
appendages also. 
In the skull of a mammal the lower jaw is all in one 
piece, instead of being obviously made up of a number 
of separate bones as it is in reptiles, etc., and, moreover, 
the mandible articulates with a large bone in the skull 
known as the squamosal, and not with a special small 
bone termed in reptiles and amphibia, the quadrate. 
At the end of the skull, where it articulates with the 
II 
