The horns serve as a fair index of the age of a bison.  After he is three years old, the bison adds each year a ring around the base of his horns, the same as domestic cattle.  If we may judge by this, the horn begins to break when the bison is about ten or eleven years old, and the stubbing process gradually continues during the rest of his life.  Judging by the teeth, and also the oldest horns I have seen, I am of the opinion that the natural life time of the bison is about twenty-five years; certainly no less.

+------------------------------------------------------

--+
|                   BISON AMERICANUS.                    |
|                (Male, eleven years old.                |
|           Taken December 6, 1866.  Montana.)            |
|       (No. 15703, National Museum collection.)       |
+--------------------------------------------------------+
|                                          |Feet.|Inches.|
|Height at shoulders to the skin           |  5  |   8   |
|Height at shoulders to top of hair        |  6  |  --   |
|Length, head and body to insertion of tail| 10  |   2   |
|Depth of chest                            |  3  |  10   |
|Depth of flank                            |  2  |   0   |
|Girth behind fore leg                     |  8  |   4   |
|From base of horns around end of nose     |  3  |   6   |
|Length of tail vertebræ                   |  1  |   3   |
|Circumference of muzzle back of nostrils  |  2  |   2   |
+--------------------------------------------------------+
pre>
8. The Cow in the third year.—­The
young cow of course possesses the same youthful appearance
already referred to as characterizing the “spike”
bull.  The hair on the shoulders has begun to take
on the light straw-color, and has by this time attained
a length which causes it to arrange itself in tufts,
or locks.  The body colors have grown darker,
and reached their permanent tone.  Of course the
hair on the head has by no means attained its full
length, and the head is not at all handsome.
The horns are quite small, but the curve is well defined,
and they distinctly mark the sex of the individual,
even at the beginning of the third year.
+------------------------------------------------------

------+
|                     BISON AMERICANUS.                      |
|(Young cow, in third year.  Taken October 14, 1886.  Montana.)|
+-----------------------------------------------------------

-+
|         (No. 15686, National Museum collection.)         |
+-----------------------------------------------------------

-+
|                                          |Feet.| Inches.   |
|Height at shoulders                       |  4  |    5      |
|Length, head and body to insertion of tail|  7  |    7      |
|Depth of chest                            |  2  |    4      |
|Depth of flank                            |  1  |    4      |
|Girth behind fore leg                     |  5  |    4      |
|From base of horns around end of nose     |  2  |    81/2     |