_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 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.[38] 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_[39] 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;[40] 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