_Dentes primores ²⁄₂; inferioribus acutis, gracilibus, et anticè
lævibus; superioribus gracilibus, anticè longitudinalitèr sulcatis._
_Molares utrinque ³⁄₃ radicati; primo maximo, ultimo minimo: primo
superiore plicas vitreas duas externè et internè alternatìm
exhibente; secundo, et tertio, plicas duas externè, internè unam:
primo inferiore plicas vitreas tres externè, duas internè; secundo,
plicas duas externè, unam internè; tertio unam externè et internè,
exhibentibus._
_Artus inæquales: antipedes 4–dactyli, cum pollice exiguo: pedes
postici 5–dactyli, digitis externis et internis brevissimis._
_Ungues parvuli et debiles. Tarsi subtùs pilosi._
_Cauda mediocris, pilis brevibus adpressis instructa._
_Caput magnum, fronte convexo: oculis magnis: auribus mediocribus._
The present genus according to my views belongs to the family _Muridæ_.
The modifications of structure which have led me to separate it from the
genus _Mus_ are as follows:
_External characters._—The most conspicuous points of distinction
between the external characters of _Reithrodon_ and _Mus_ (if we
regard _M. rattus_, _M. decumanus_ or _M. musculus_ as typical
examples of that genus,) consist in the arched form of the head, the
large size of the eyes, the stout form of the body, and the upper
incisors being grooved. The ears, tail and feet are more densely
clothed with hairs, and the tarsus is covered with hair beneath,—at
least the hinder portion.
_Cranium._—The skulls of the species of the present genus differ from
those of the species of _Mus_ in being proportionately shorter and
broader, and more arched; the facial portion of the skull is larger,
compared with the cranial, the space between the orbits is narrower,
and the orbits are larger; the palate is narrower and the incisive
foramina are more elongated and larger. The pterygoids approximate
anteriorly, so that the posterior _nares_ are greatly contracted. As
in the genus _Mus_ the anterior root of the zygomatic arch is
directed upwards from the plane of the palate, and forwards in the
form of a thin plate, protecting an opening behind, which leads into
the nasal cavity, and also forming the outer boundary both of the
ant-orbital foramen, and a second opening whose outlet is directed
upwards. This thin plate, however, is narrower than is usually found
in the genus _Mus_. The most striking differences observable in the
lower jaw consist in the smaller size of the coronoid process, and
its being curved outwards; the condyloid process is narrower, and
the angle of the jaw, or descending ramus, approaches more nearly to
a quadrate form—the posterior edge of the jaw is more deeply
emarginated.
_Dentition._—The incisors are narrow and compressed as in the genus
_Mus_, but they are less deep from front to back; those of the upper
jaw (Plate 33. fig 2. _b._) have each a distinct longitudinal
groove, which is situated nearer to the outer than to the inner edge
of the tooth. Close to the inner edge of each of these teeth an
indistinct second longitudinal groove may be seen by means of a
lens. The lower incisors are nearly equal in width to the upper.
The crowns of the molar teeth in the young _Reithrodon_ are higher
than in _Mus_, and they are rootless; in the adult animal, however,
they possess distinct roots. The folds of enamel form sigmoid
flexures, are closely approximated to each other, and those of the
opposite sides of the tooth meet.
1. REITHRODON CUNICULOÏDES.
PLATE XXVI.
Reithrodon cuniculoïdes, _Waterh._, Proceedings of the Zoological
Society of London for February 1837, p. 30.
_R. suprà griseus, flavo-lavatus, pilis nigris intermixtis; abdomine
guláque pallidè flavis; natibus albis; pedibus albis; auribus
mediocribus, intùs pilis flavis, extùs pilis pallidè flavis,
obsitis; pone aures, notâ magnâ albescenti-flavâ; caudâ corpore
breviore, suprà pallidè fuscâ, subtùs albâ._
DESCRIPTION.—Head rather large and arched; ears moderate; tail nearly
as long as the body; tarsi rather long; fur long and very soft.
General tint of the upper parts of the body grayish brown, with a
considerable admixture of yellow; of the sides of the body grayish
tinted with yellow; the lower portion of the cheeks, and the lower
half of the sides of the body are of a delicate yellow colour; the
under parts of the head and body are yellowish white; the fore
part of the thighs is whitish; the rump, feet, and tail are white,
excepting the upper surface of the latter, which is brown; behind
each ear there is a patch of yellowish white hairs. The ears are
tolerably well clothed with hairs; those on the inner side are for
the most part of a yellow colour, but towards the posterior margin
they are brown; externally, the hairs are also yellow, excepting
on the fore part, where they are dusky brown. The hairs of the
moustaches are very long and numerous; black at the base, and
grayish at the apex. The feet are well clothed with hairs which
cover and nearly hide the claws; the under side of the tarsus is
clothed with grayish brown hairs. The tail is well clothed with
tolerably long hairs which completely hide the scales. The hairs
on the back are of a deep gray colour at the base, broadly
annulated with yellow near the apex, and dusky at the apex: the
longer hairs are black: on the throat and belly the hairs are deep
gray at the base, and broadly tipped with pale yellow—towards the
cheeks and sides of the body with a deeper yellow. The incisors
are yellow.
In. Lines.
Length from nose to root of tail 6 5
of tail 3 3½
from nose to ear 1 4
of tarsus (claws included) 1 4¼
of ear 0 7
Habitat, Patagonia, (_April and January_).
In the arched form of the head this little animal bears considerable
resemblance to a young rabbit, a resemblance which has struck almost all
who have seen it, I have therefore applied to it the specific name
_Cuniculoïdes_. The skull is figured in Plate 33, fig. 2. _a._, its
dimensions are as follows:—
In. Lines.
Total length 1 4
Width 0 10
Length of nasal bones 0 7
of incisive foramina 0 4¾
Distance between the outer surface of the incisors and the
front molar upper jaw 0 5
Longitudinal extent of the three molars of the upper jaw 0 3¾
Length of a ramus of the lower jaw, without the incisor 0 9¾
The molar teeth of the upper jaw are figured in Plate 33, fig. 2, _c._
and 2, _e_; of the lower jaw, fig. 2, _d_. Fig. 2, _b_, represents the
incisors of the upper jaw magnified. Fig. 21, _a_, Plate 34, represents
the skull, viewed from beneath, fig. 21, _b_, is the side view of the
same, and fig. 21, _c_, is the lower jaw.
“Specimens were procured at Port Desire, St. Julian, and Santa Cruz; at
this latter place they were caught in numbers, (in traps baited with
cheese,) both near the coast and on the interior plains. A specimen from
Santa Cruz weighed 1336 grains. In the early part of January, there were
young individuals at Port St. Julian.”—D.