FEB., 1912. MAMMALS OF ILLINOIS AND WISCONSIN 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. 



Length of tail 



Length of hind foot 



LENGTH Distance from end of nose to end of vertebras of tail (not 

 including the hairs which may extend beyond) . 

 * For definitions of terms, see Glossary, p. 489. 



