FEB., 1912. MamMats OF ILLINOIS AND WIScoNnsIN — Cory. 37 
HOW TO ESTIMATE THE AGE OF A MAMMAL.* 
For purposes of identification it is often essential for the student to 
know whether an animal is old or young, as many species vary greatly 
in appearance at different ages. In some cases he will learn to recognize 
immature specimens by their smaller size and the difference in the color 
of their pelage; but in others, where the pelage differs but little, or the 
young animal has nearly attained its full growth, an approximate idea 
of its age can best be formed by reference to anatomical characters, 
such as the degree of ossification and anchylosis of the epiphyses, the 
condition of the cranial sutures and of the teeth. 
In the young animal the epiphyses are cartilaginous, but in most 
cases they gradually ossify and ultimately (usually at the age of matu- 
rity) become anchylosed to the main part of the bone. The cranial 
sutures, which in the immature animal have the appearance of “‘cracks’”’ 
in the skull, gradually become anchylosed, and in advanced age their 
line of union is often but faintly indicated and in many cases, by the 
ossification of the intervening tissue, become entirely obliterated. If 
the edges of the bones are notched, so as to interlock, the union is called 
a dental suture; but, if they are bevelled and overlap, the union is a 
squamous suture. 
Deciduous teeth, or ‘“‘milk teeth,” are present in the young in most 
species; later they are replaced by permanent teeth. At the age of 
maturity the permanent teeth are usually in perfect condition, but in 
old specimens they are much worn. 
HOW TO MEASURE A MAMMAL. 
Total length 
) 
Lrenoru — Distance from end of nose to end of vertebre of tail (not 
including the hairs which may extend beyond). 
*For definitions of terms, see Glossary, p. 489. 
