to the front of the incisor, so that in gnawing the softer dentine is worn away at the tip faster than the enamel, thus keeping the tooth sharp. There are no canine teeth. Rodents are mostly herbivorous in diet, are usually small, and have clawed digits. The family Muridae, to which rats and mice belong, is said to include nearly half the genera of the Rodentia.
Four common species of this family are found in the United States. The common house mouse (Mus musculus) is less than two hundred millimeters in length when adult. The generic designation Epimys has been established for the rats, but the older term Mus is doubtless more familiar. Mus rattus rattus (the black, or house rat) is the rat of the Middle Ages in Europe. It probably reached that continent some six hundred years before the Norway rat arrived. The black rat preceded the Norway rat in North America, and occurs still in scattered localities in the northern United States. Its diagnostic characteristics are: the ears when laid forward extend at least to the middle of the eye; the tail is longer than the head and body; and is dusky all around; hind foot 33–37 millimeters; the color is blue-black above, and slaty below. A color variety, Mus rattus alexandrinus (roof, snake, or Alexandrian rat) is "grayish-brown above, white or yellowish white below" (Lantz). It is said to be common in the southern states near the coast. Mus norvegicus (the brown, gray, barn, wharf, sewer, or Norway rat) has, as a result of its adaptability, fecundity, strength, and ferocity, largely replaced the other species of rats in temperate climates. When well fed it is said that a female Norway rat can give birth to from sixty to a hundred young a year. It is more of a burrower than the black or roof rat, and is the common form found about bams, houses, warehouses, etc., in the United States. This species' characteristics are: "Ears moderate,
