316 MAMMALIA : GLIRES, — XLVII. 
Famuy CLXXX. DIDELPHIDIDA#. (Tax Opossums.) 
Marsupial mammals of small size, with the teeth i. $:4, c. 4:4, 
pm. $78, m. 4:4. Feet five-toed, plantigrade, the claws 5-4. Tail 
usually very long , prehensile, nearly naked, covered by a scaly skin, 
with a few scattered hairs. Genera 2, species about 15; all Ameri- 
can and chiefly belonging to the tropics. The common opossum 
is one of the largest of the group. All are sluggish animals, 
arboreal (Didelphis) or aquatic (Chironectes), and becoming very 
fat. They feed on insects and other small animals. 
a. Arboreal; feet not webbed. . . . + « « « + Dre pms, 536. 
536. DIDELPHIS Linneus. (8is, two; dedgvs, womb.) 
1029. D. virginiana Kerr. Common Opossum. Soiled yel- 
lowish, with some darker hairs; ears black, leathery ; legs dark. 
L. 35. T. 15. N. Y. to Cal. and S.; common. 
Orprr XLVID. GLIRES. (Tue Ropents or GNaweErs.) 
Mammals with the incisor teeth 4 or 3 in number, chisel-shaped, 
adapted for gnawing; no canine teeth, a toothless space in the 
place of canines; molar teeth adapted for grinding; cerebrum 
small, little convoluted; intestinal canal elongate; ears and eyes 
usually well developed. Food chiefly vegetable. 
The Glires or Rodentia is the largest order of Mammals, and in 
individuals by far more numerous than any other. Most of the 
species are of small size, the Beaver being one of the very largest 
of the forms now living. 
“Though a feeble folk, comparatively insignificant in size and 
strength, they hold their own in legions against a host of ‘natural 
enemies, rapacious beasts and birds, by their fecundity, their wari- 
ness and cunning, their timidity and agility, their secretiveness, 
each after the means by which it is provided for exercising its in- 
stinct of self-preservation, among which insignificance itself is no 
small factor.” (Coues.) (Lat., glis, dormouse ; the Linnzan name 
Glires is much older than Cuvier’s Rodentia.) 
Families of Glires. 
a. Incisors $, the median upper incisors large, vertically grooved, the outer 
small; teeth 28; tail very ee ears long; fibula united with the heel- 
DME cas 4 «+ « » » « « Lepvormw4, 181. 
aa. Incisors # ; tail well developed. 
b, Fur with stiff spine-like bristles; tibia and fibula separate. 
ERETHIZONIDZ, 182. 
6b. Fur more or less soft, without spines. 
ce. Tibia and fibula united below. 
d. Tail and hind legs excessively elongated, the latter adapted for 
leaping; molars 4 on each side. + oe . 6 « ZAPODID., 183, 
