in both families we have long-tailed and short-tailed forms. Cercolabes corresponds to Atherura or Trichys, and Erethizon to Hystrix.
The genus Erethizon, the "Urson" of Canada, has a short, stumpy tail. Its spines are almost hidden by enveloping hair. The fore-feet have four, the hind-feet five toes. The short tail of this creature is remarkable when we reflect upon its climbing habits. It appears, however, to be a weapon with which it strikes sideways at the enemy.
Fig. 244.—Brazilian Tree Porcupine. Sphingurus prehensilis. × 1⁄6.
Of the Neotropical genus Cercolabes (sometimes called Sphingurus, Synetheres, or Coendou) there are some eight or nine species, all found in Central and South America. The animal is arboreal, and has in correspondence with that habit a prehensile tail. The spines are not so stout as in the Ground Porcupines, and are often coloured yellowish or reddish. In correlation with its tree-frequenting habits the bones of Cercolabes show certain differences from those of the Ground Porcupines. The scapula is broader and rounder in front than is that of Hystrix; the phalanges of the thumb (which is rudimentary) are fused together as in the Canadian Erethizon; but those of the very small hallux are also fused, whereas in Erethizon, as in Hystrix, they are separate. In one species, C. insidiosus, Sir W. Flower states that there are as many as seventeen dorsal vertebrae and thirty-six caudals. The tail is thus very long. In C. villosus there are fifteen dorsals and twenty-seven caudals; eight ribs reach the sternum, which is composed of seven pieces, the
sixth being very small. The clavicles are well developed. A curious fact about C. villosus is that the acetabular cavity is perforate (on both sides), or at least only closed by membrane. In many forms of Rodents the bone is very thin in this region. This fact perhaps lessens the significance of the perforation of the acetabulum of Echidna (see p. [109]).
Of the allied genus Chaetomys, also Neotropical, there is but a single species, which inhabits Brazil. It has a nearly completely closed orbit, a feature which differentiates it from the last animal, and one which also shows it to be a more modified form. The spiny covering is less pronounced than in its allies.
Fam. 8. Hystricidae.—This family is characterised by the fact that all its members possess spines; but the tail, if at all long, is not prehensile, and the soles of the feet are smooth and not covered with rough tubercles, as in the Tree Porcupines of the next family, Erethizontidae. The clavicle is less developed than in the arboreal forms. In the organs of digestion there are points of a family difference between the two groups of spiny Rodents. The tongue has serrated scales arranged in transverse rows, which are directed backwards. A gall-bladder, though not always present, is sometimes found; it apparently never exists in the arboreal Porcupines and in Erethizon. The lungs show a great tendency to subdivision, which appears to be especially marked in the genus Atherura. The caecum seems also to be shorter in the Ground Porcupines. In Hystrix cristata the small intestine measures 15 feet 7 inches; the caecum, 8 inches; the large intestine, 4 feet 4 inches:—in Atherura africana the caecum measures 7½ inches; the large intestine, 1 foot 10 inches. The corresponding measurements of Synetheres villosus were: small intestine, 7 feet 3 inches; caecum, 1 foot 4 inches; large intestine, 2 feet 7 inches. In Erethizon the caecum is 2 feet 4 inches in length. These differences are too large and too constant in a number of presumedly allied forms to be overlooked.
Mr. Parsons has directed attention[[359]] also to a number of muscular differences, such indeed as might be expected to occur between animals of such different habits.
The genus Hystrix embraces the better-known Porcupines. It is a genus of wide range, extending from the East Indies to Africa,