It is customary and convenient to express the numbers and kinds of teeth of a given mammalian species by means of a “dental formula”; for example, in Man the formula is: _i_ 2/2, _c_ 1/1, _p_ 2/2, _m_ 3/3, × 2 = 32; the reason for the multiplication by two is that the figures deal only with one side of the mouth and must be doubled to give the sum total. Just because, however, the two sides are alike, it is usual to take the doubling for granted. Written out in full, the formula means that Man has two incisors, one canine, two premolars and three molars on each side of each jaw, the horizontal line indicating the division between upper and lower teeth. The number of teeth is frequently not the same in the upper and lower jaws; for instance, the formula for the Sheep is: _i_ 0/3, _c_ 0/1, _p_ 3/3, _m_ 3/3, × 2 = 32; the total is the same as in Man, but the arrangement is entirely different. The meaning is that in the Sheep there are no upper incisors or canines, but three incisors and a canine are present in each half of the lower jaw, with three premolars and three molars on each side above and below. The Dog gives still another formula: _i_ 3/3, _c_ 1/1, _p_ 4/4, _m_ 2/3, × 2 = 42. What is called the typical formula for the higher terrestrial mammals above the grade of the marsupials and which is but rarely exceeded, is _i_ 3/3, _c_ 1/1, _p_ 4/4, _m_ 3/3, × 2 = 44, though most existing mammals have fewer teeth than this. Compared with the typical formula, the Dog has lost but two teeth, the third upper molar on each side, while Man and the Sheep have each lost twelve.