Part 27

Vellus perlongum, et molle.

The genus Abrocoma is evidently allied on the one hand to the genera Octodon, Poephagomys, and Ctenomys, and on the other to the family Chinchillidæ. The four genera just mentioned possess so many characters in common, that it would be well to unite them, and the name Octodontidæ may be used to designate the group.

The Octodontidæ appear to bear the same relations to Echimys, as the Arvicolæ do to the Muridæ.

In the Octodontidæ the skull is rather short, the inter-orbital space is broad; the ant-orbital passage is large; the zygomatic arch is thrown out horizontally from the plane of the palate; the malar bone is broad and somewhat compressed, and throws up a small post-orbital process; the glenoid cavity of the temporal bone is narrow; the palate is contracted, and deeply notched posteriorly, the portion which lies between the molar teeth descends below the level of the anterior portion; the incisive foramina are wide: the body of the anterior and posterior sphenoids is very narrow, and the foramina on either side of them are large: the occipital condyles are very narrow, widely separated, and the articular surface is nearly vertical. The descending ramus of the lower jaw springs from the outer side of the alveolar portion, and terminates in a point, more or less acute.

The incisors of the upper and lower jaws are of the same width: the molars are ⁴⁄₄=⁴⁄₄, rootless.

In external characters the species of the present group vary considerably. The toes are ⁵⁄₅ or ⅘. The claws of the hind feet are covered by strong, curved bristly hairs.

The principal points of distinction in the external characters of the four genera under consideration, may be thus expressed.

† TOES ⁵⁄₅.

A. Fore feet formed for burrowing—strong and armed with large claws; tail short. a. Ears minute, incisors very broad Ctenomys. b. Ears small, incisors broad Poephagomys.

B. Fore feet weak; claws small; incisors narrow; ears large. a. Tail with the apical portion furnished with long hair Octodon.

†† TOES ⅘.

b. Tail furnished throughout with short adpressed hairs Abrocoma.

It is not only in the comparatively small size and weakness of the fore feet that Abrocoma approaches more nearly to Octodon; but it agrees in having the soles, both of the fore and hind feet (which are devoid of hair), covered with minute round fleshy tubercles (see the under side of the tarsus figured in Plate 28.)

In Octodon, however, the toes have on their under side transverse incisions, as the Muridæ, and many other Rodents; a character not found in Abrocoma. Here the underside of the toes, like the sole of the foot, is covered with minute tubercles.

Though in the form of the skull Abrocoma Cuvieri agrees most nearly with that of Octodon; it differs in having the anterior portion narrower and rather larger, compared to the part devoted to the protection of the brain; the zygomatic arch is shorter, the incisive foramina are longer, the body of the anterior sphenoid is narrower, and the auditory bullæ are larger. The principal differences observable in the form of the lower jaw of Abrocoma, when compared with that of Octodon, consists in the coronoid process being smaller, the condyloid narrower from front to back; the descending ramus more deeply emarginated posteriorly, and the angle longer and more attenuated.

In those characters in which the skull of Abrocoma departs from that of Octodon, it approaches nearer to Chinchilla. In the peculiar form and large size of the ears, in the extreme softness of the fur, in the greater development of the pads on the under side of the toes, and in the possession of only four toes to the fore feet, there are other points of resemblance between Abrocoma and Chinchilla. In the Chinchilla as well as in Octodon and Abrocoma, we find the toe corresponding to the second (counting from the inner side) furnished with a broad hollow nail; there are also stiff bristly hairs covering this nail as in the Octodontidæ.

The extreme softness of the fur of the animals about to be described, suggested for them the generic name of Abrocoma. The fur consists of hairs of two lengths, and the longer hairs are so extremely slender that they might almost be compared to the web of the spider. The specific names applied are those of the distinguished naturalists who first made us acquainted with the two genera, Octodon and Poephagomys.

1. ABROCOMA BENNETTII. PLATE XVIII.

Abrocoma Bennettii, Waterh., Proceedings of the Zoological Society of London, for February 1837, p. 31.

A. corpore suprà griseo, ad latera pallidiore et pallidè cervino lavato, subtùs albescenti-cervino; gulâ albescenti-griseâ; pedibus sordidè albis: auribus amplis, ad marginem posticum rectis, extùs ad bases vellere, sicùt in corpore, obsitis: caudâ corpore breviore, ad basin crassiusculâ, pilis brevibus incumbentibus vestitâ.

DESCRIPTION.—Form stout; ears large, with the posterior margin straight; fore feet rather small, tarsus short; tail rather shorter than the body, thick at the base; fur long and extremely soft, and silk-like. General colour pale grayish brown, with a slight yellow wash; the upper part of the head and the back dusky brown; under parts of the body very pale yellowish brown, inclining to white; chin and throat whitish; feet dirty white; tail well clothed with hairs, which are closely adpressed, brown above, and of a very pale brown beneath at the base, darker towards the apex. The hairs of the moustaches are numerous, long, rather slender, and of a brownish colour. The ears are brown, furnished externally at the base with fur resembling that of the body; the remaining parts (both external and internal) are beset with long and extremely slender brown hairs, which project considerably beyond the margin of the ear. The ordinary fur on the back is about ten lines in length, but thickly interspersed with this fur, are longer hairs which are so delicate that they may almost be compared to the spiders’ thread. Both on the upper and under side of the body the fur is deep gray at the base. The incisors are yellow.

In. Lines. Length from nose to root of tail 9 9 of tail 5 0 from nose to ear 1 11 of tarsus (claws included) 1 4 of ear 0 10 Width of ear 1 0½

Habitat, Chile, (August.)

“This animal was caught amongst some thickets in a valley on the flanks of the Cordillera, near Aconcagua. On the elevated plain, near the town of Santa Rosa, in front of the same part of the Andes, I saw two others, which were crawling up an acacia tree, with so much facility, that this practice must be, I should think, habitual with them.”—D.

2. ABROCOMA CUVIERI. PLATE XXIX.

Abrocoma Cuvieri, Waterh., Proceedings of the Zoological Society of London for February 1837, p. 32.

Ab. suprà grisea, levitèr ochraceo lavata; abdomine gulâque albescenti-griseis; pedibus sordidè albis; auribus amplis, ad marginem posticum distinctè emarginatis; caudâ corpore multò breviore, et nigrescente.

DESCRIPTION.—Ears large; tail considerably shorter than the body; fur extremely soft; general colour gray faintly washed with yellow; under parts of the body grayish white; feet dirty white; tail dusky, paler beneath at the base: the ears are large, distinctly emarginated behind, and appear to be almost naked, but, upon close examination, long and extremely fine hairs may be observed. All the fur is gray at the base; the hairs of the moustaches are numerous and very long, those nearest the mouth are white, the others are black at the base and grayish beyond. The incisors are of a palish yellow colour.