Genus—ABROCOMA.[37]
Dentes primores ²⁄₂ acuti, eradicati, anticè læves: molares utrinque ⁴⁄₄ subæquales, illis maxillæ superioris in areas duas transversales ob plicas vitreas acutè indentatus divisis; plicis utriusque lateris vix æquè profundis; illis mandibulæ inferioris in tres partes divisis, plicis vitreis his internè, semel externè indentatis, areâ primâ sagittæ cuspidem fingente, cæteris acutè triangularibus.
Artus subæquales.
Antipedes 4–dactyli, externo brevissimo, intermediis longissimis et ferè æqualibus.
Pedes postici 5–dactyli; digito interno brevissimo. Ungues breves et debiles, illo digiti secundi lato et lamellari; omnibus setis rigidis obtectis.
Caput mediocre, auribus magnis, membranaceis; oculis mediocribus.
Cauda breviuscula.
Vellus perlongum, et molle.
The genusAbrocomais evidently allied on the one hand to the generaOctodon,Poephagomys, andCtenomys, and on the other to the familyChinchillidæ. The four genera just mentioned possess so many characters in common, that it would be well to unite them, and the nameOctodontidæmay be used to designate the group.
TheOctodontidæappear to bear the same relations toEchimys, as theArvicolædo to theMuridæ.
In theOctodontidæthe 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.[38]The descendingramusof 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.
It is not only in the comparatively small size and weakness of the fore feet thatAbrocomaapproaches more nearly toOctodon; 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.)
InOctodon, however, the toes have on their under side transverse incisions, as theMuridæ, and many other Rodents; a character not found inAbrocoma.Here the underside of the toes, like the sole of the foot, is covered with minute tubercles.
Though in the form of the skullAbrocoma Cuvieri[39]agrees most nearly with that ofOctodon; 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 ofAbrocoma, when compared with that ofOctodon, consists in the coronoid process being smaller, the condyloid narrower from front to back; the descendingramusmore deeply emarginated posteriorly, and the angle longer and more attenuated.
In those characters in which the skull ofAbrocomadeparts from that ofOctodon, it approaches nearer toChinchilla. 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 betweenAbrocomaandChinchilla. In the Chinchilla as well as inOctodonandAbrocoma, we find the toe corresponding to the second (counting from the inner side) furnished with a broad hollow nail;[40]there are also stiff bristly hairs covering this nail as in theOctodontidæ.
The extreme softness of the fur of the animals about to be described, suggested for them the generic name ofAbrocoma. 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,OctodonandPoephagomys.
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; forefeet 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.
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.
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 extremelysoft; 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.
Habitat, Chile, (September.)
This species is about one-third the size of the last, it differs moreover in being gray instead of brown, and in having the posterior margin of the ear emarginated; the tail is also rather shorter in proportion.
The skull[41]is figured in Plate 33, fig. 1,a, and 1,b; and fig. 23,a, Plate 34. Its length is 1 inch, 4½ lines; width 9¼ lines; length of nasal bones 6 lines; distance between fore part of incisors and the front molar (upper jaw) 5 lines; longitudinal extent of the three molars of upper jaw 3 lines; length of auditory bullæ 5¾ lines; length oframusof lower jaw (see Plate 33, fig. 1,c,), without incisors, 11½ lines. Fig. 23,c, Plate 34, represents the inner side of aramusof the lower jaw: fig. 1,d, Plate 33, is the lower jaw seen from above: fig. 23,b, Plate 34, is the same seen from beneath. This view is given to show the position of the descending ramus of the lower jaw—that it springs from the outer side of the alveolar portion, as in a great portion of the South American Rodents, such asDasyprocta,Myopotamus,Echimys,Chinchilla, and also in that genus found in the West Indian islands,Capromys. Fig. 1,e, Plate 33, represents the molar teeth of the upper jaw, and fig. 1,f, those of the lower.
“This species is abundant on the dry hills, partly covered with bushes, near Valparaiso.”—D.