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 are, 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 



