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Rh than one pair of premolars, and the original ridges of all the cheek-teeth have become obscured and complicated by the development of secondary enamel-folds. The majority of these rodents, many of which are of large size, are terrestrial, but a few are burrowing, others arboreal and two or three aquatic.

The Old World porcupines, constituting the family Hystricidae, are terrestrial, stoutly built rodents, with limbs of subequal length in front and behind, and the skin covered with strong spines. The upper lip is cleft, the jugal lacks an inferior angle, the fore part of the skull is short and broad; the cheek-teeth are partially rooted, with external and internal enamel-folds, the soles of the feet are smooth, there are six pairs of teats, the clavicles are imperfect and the tail is not prehensile. In the typical genus Hystrix, which is represented in all the three great continents of the Old World, and extends as far east as Flores and Celebes, the skull is swollen and convex, the spines are cylindrical, and the tail is short and covered with spines and slender-stalked open quills. In Atherura fasciculata of the Malay Peninsula the spines are flattened, and the tails long and scaly, with a tuft of compressed bristles. A closely allied species, A. africana, inhabits Western Africa. The third genus is Trichys (see ).



American Porcupines.—All the New World porcupines, representing the family Erethizontidae (or Cöendidae) are arboreal in their habits, and have the upper lip undivided, the cheek-teeth rooted, the clavicles complete, the soles of the feet tuberculated and three pairs of teats. Erethizon dorsatus, the urson, is distributed all over the forest regions of North America; Synetheres (or Cöendu) prehensilis, the prehensile-tailed porcupine of South America (fig. 15), represents a genus in which the whole upper surface of the body is protected by long white-tipped spines; Chaetomys subspinosus is clothed with strong wavy bristles. In the last two genera the feet have four toes, in place of the five of Erethizon (see ).

Cavy Group.—In the family Caviidae, typified by the cavies (or guinea-pigs), may be included a large number of South and Central American rodents, among which the agoutis and pacas are often ranked as a family (Dasyproctidae) by themselves. The Caviidae, in the present more comprehensive sense, include the giants of the rodent order. Many of them, like ungulates, are specialized for swift running, and have unusually long limbs, with ridges developed on the articular surfaces of the lower bones; the clavicles are more or less reduced; the thorax is more compressed than usual, with a narrower breast-bone; and there is a marked tendency to the reduction or loss of the lateral toes, more especially in the hind limb. Since these rodents walk more or less entirely on their toes, in such a manner that the edges of the claws or nails come in contact with the ground, these tend to assume somewhat of a hoof-like character; while the foot-pads are more or less horny. The tail is generally very short, and its basal vertebrae are often fused with the sacrum. In the skull the lachrymal bone is large, the par-occipital process is directed vertically downwards and the tympanic bulla is hollow. In the soft parts the caecum is very large, the penis is armed with a pair of barbed horny claspers and the scrotum is spiny.

Special interest attaches to the most aberrant member of the

family, the Peruvian Dinomys, known for more than thirty years only by a single specimen taken in a house in Lima, and only lately rediscovered. It is a large rodent known to the Tupi Indians as the paca-rana, or false paca, in allusion to the resemblance of its coloration to that of the true paca, from which it differs by its well-developed tail, the absence of cheek-pouches, the full development of all live toes and the wider thorax. The Tupi name may be adopted as the popular title of the species. Dr E. Goeldi states that the paca-rana is a rodent of phlegmatic and gentle disposition, which may account, perhaps, for its rarity, if, indeed, it be really scarce in its native home, which is probably the eastern slopes and tablelands of the Bolivian and Peruvian foot-hills bordering on Brazil, inclusive of the headwaters of the Purus, Acre and Jurua rivers. In the true pacas, Coelogenys (or Agouti), the first front toe is small, and both the first and fifth digits of the hind-foot are much inferior in size to the other three. The most remarkable feature of the genus is, however, the extraordinary development of the zygomatic arches of the skull, which are enormously expanded vertically, forming great convex bony capsules on the sides of the face, enclosing on each side a large cavity lined with mucous membrane internally, and communicating by a small opening with the mouth. C. paca is a white-spotted rodent, about 2 ft. long, and lives generally in the forests or along the banks of rivers (see ). The Agoutis, Dasyprocta, include several species of slender-limbed rodents, with three hind-toes, inhabiting Central and South America, one (D. cristata) extending into the West Indian islands. The members of both Coelogenys and Dasyprocta are terrestrial in their habits, and have the fore- and hind-limbs subequal, hoof-like claws, short or obsolete tail and rudimentary clavicles. The masseteric ridge of the lower jaw is obsolete, the palate broad, the incisors long and the molars semi-rooted, with external and internal enamel-folds (see ). The remaining and more typical members of the family, one of which is aquatic, are characterized by their short incisors, the strong masseteric ridges on the sides of the lower jaw, the long and curved par-occipital and the palate contracted in front. Fore-feet with four digits, hind-feet with three; clavicles imperfect; molars divided by enamel-folds into transverse lobes; milk-teeth shed before birth. In the true cavies, or couies, Cavia, the fore- and hind-limbs are short and of subequal length, the ears are short and there is no tail. They include several species widely distributed throughout South America, extending even to the straits of Magellan, from one of which (C. cutleri of Peru) the guinea-pig is derived. The maras (Dolichotis) have the limbs and ears long and the tail very short. D. patagonica is a large species, nearly 3 ft. long, inhabiting the gravelly plains of Patagonia, while D. salinicola is a much smaller rodent from the salt-lagunas of Argentina. The palate is so much contracted in front that the premolars of opposite sides touch by their antero-internal edges. Hydrochaerus, in which all the feet are fully webbed, includes a single species, the capybara, or carpincho, the largest of living rodents. The skull (fig. 14) is distinguished not only by its great size, but by the enormous development of the par-occipital processes and the complex structure and large size of the last molars (see and ).

Chinchilla Group.—The family, Chinchillidae, typified by the well-known chinchilla, includes a small number of South American rodents with large ears and proportionately great auditory bullae in the skull, elongated hind-limbs, bushy tails, very soft fur and perfect clavicles. The jugal is without an inferior angle, and extends forwards to the lachrymal; the palate is contracted in front and deeply emarginate behind; the incisors are short, and the molars divided by continuous folds into transverse plates; and the two halves of the lower jaw are welded together in front. It includes three existing genera, represented by some five species. Of these the true chinchilla, Chinchilla lanigera, C. brevicaudata, Lagidium peruanum and L. pallipes, are restricted to the alpine zones of the Andes from the northern boundary of Peru to the southern parts of Chili; while Lagostomus trichodactylus (or Viscaccia viscaccia), the viscacha, is confined to the pampas from the Uruguay river to the Rio Negro. In Chinchilla the fore-feet have five and the hind four digits, the tail is long and bushy, and the auditory bullae are enormous, appearing on the top of the skull; Lagidium has four digits in both fore- and hind-feet, and Lagostomus three only in the hind-feet, while the auditory bullae are much smaller (see and ).

Hutia Group.—The three remaining families of the Hystricoidea, of which one is African while the other two are chiefly South American, are very closely allied and often brigaded in a single family group. In the Capromyidae, which includes only the South American and West Indian hutias, the South American coypu and the African cane-rats, the tympanic bulla of the skull is hollow, the par-occipital process straight, the lachrymal small, and the cheek-teeth rooted, with deep enamel-folds; the first front toe being occasionally absent. Of the few living representatives of the group, the genus Myocastor (or Myopotamus) is represented only by the South American coypu, M. coypu, which is aquatic in its habits, and measures about 2 ft. in length, being the largest member of the group. It has a long tail, brown fur and red incisors, and lives in burrows near water, feeding on aquatic plants.