Reithrodon typicus, _Waterh._, Proceedings of the Zoological Society
of London for February 1837, p. 30.
_R. vellere suprà pilis flavescenti-fuscis et nigrescentibus
intermixtis composito; regione circa oculos, genis, lateribusque
corporis auratis, pilis pallidè fuscis intermixtis; partibus
inferioribus auratis; rhinario ad latera flavescenti-albo; auribus
magnis, intùs pilis flavis, extùs flavis et fuscis, indutis; caudâ
suprà pallidè fuscâ, subtùs sordidè albâ; pedibus albis._
DESCRIPTION.—Ears large; tarsi moderate; fur moderately long; general
tint of the upper parts brown—of the upper surface of the head
blackish; on the cheeks and flanks a rich yellow tint prevails;
the under parts of the head and body are bright yellow; the feet
are white; the tail is brownish above and dirty white beneath. The
ears are tolerably well clothed with hairs, and these are of a
yellowish colour, excepting on the fore part, externally, where
they are brown; the tarsi are covered beneath with grayish brown
hairs; the hairs of the moustaches are numerous and moderately
long, black at the base and grayish at the apex. The hairs of the
back are deep gray at the base, broadly annulated with yellow near
the apex, and black at the apex; on the upper surface of the head
the hairs are very narrowly annulated with yellow, hence a
blackish hue prevails. The longer hairs on the back are black; the
hairs of the throat and belly are gray at the base, and broadly
tipped with yellow. The incisors are yellow.
In. Lines.
Length from nose to root of tail 6 0
of tail ?[26]
from nose to ear 1 4½
of tarsus (claws included) 1 2½
of ear 0 8½
Habitat, Maldonado, La Plata, (_June_).
This species is of a darker colour than the last, its ears are much
larger and the tarsi are shorter. It has the same rabbit-like
appearance. The molar teeth of the lower jaw are figured in Plate 33,
fig. 4, _a_.
“This mouse, when alive, from its very large eyes and ears, had a
singular appearance, somewhat resembling that of a little rabbit. It
frequents small thickets in the open grassy savannahs near Maldonado,
and was caught with facility by means of traps baited with cheese.”—D.
3. REITHRODON CHINCHILLOIDES.
PLATE XXVII.
_R. vellere longissimo et mollissimo; corpore suprà et ad latera
cinereo, flavescenti-fusco lavato, subtùs flavescenti-albo; caudâ
corpore breviore, suprà fuscâ, subtùs albâ: auribus parvulis: tarsis
mediocribus._
DESCRIPTION.—Ears small; tail shorter than the body; tarsus moderate;
fur long and extremely soft. General hue of the upper parts of the
head and body ashy-brown; the lower part of the cheeks and sides
of the body are of a delicate yellow colour; the under parts of
the head and body and the rump are cream colour. The ears are
blackish;[27] the tail is tolerably well clothed with longish
hairs, which are, however, not so thickly set as to hide the
scales—on the upper side they are blackish brown; on the sides and
beneath they are white. The feet are white. All the fur is of a
deep gray colour at the base; the hairs of the back are of a very
pale yellow colour (almost white) near the tip, and brown at the
tip; the longer hairs are black at the apex. The incisors are
yellow; the hairs of the moustaches are numerous and very
long—some of them are whitish, and others are black at the root,
and gray at the apex.
In. Lines.
Length from nose to root of tail 5 0
of tail 2 4
from nose to ear 1 2
of tarsus (claws included) 1 0
of ear 0 5½
Habitat, South shore of the Strait of Magellan, near the Eastern
entrance.
This little animal was preserved in spirit, and has since been mounted,
it is probable, therefore, that the colours have been slightly changed.
It is of a smaller size than either of the preceding species. Its fur is
long, extremely soft, and somewhat resembles that of the Chinchilla. The
ears are smaller, and the tail is shorter, and less densely clothed with
hairs than in _Reithrodon cuniculoides_. The skull (see Plate 43, fig.
20, _a_, 20, _b_, and 20, _c_,) differs in many respects from that of
the species last mentioned. It is of a smaller size, the nasal portion
is proportionately shorter and narrower, the incisive foramina are
shorter; the pterygoid processes do not approximate so nearly at their
base, and the pterygoid fossæ are very shallow, whereas in _R.
cuniculoides_ they are deep. In the skull of the animal just mentioned
there are two distinct longitudinal grooves on the palate, which extend
backwards from the incisive foramina, and terminate in two rather large
and deep excavations: these excavations are in the palatine bone, and
situated between the last molar teeth; they are separated from each
other by a narrow, longitudinal, elevated ridge; a narrow ridge also
separates them from the pterygoid fossæ. At the bottom of each of these
hollows are several minute foramina, and in front of them there are two
larger longitudinal foramina. In _R. chinchilloides_, the longitudinal
grooves on the palate and the posterior hollows are shallow, and
consequently much less distinct; the pterygoid fossæ are very nearly on
the same plane as the palate, and are indicated only by a very slight
depression. The incisor teeth are broader than in _R. chinchilloides_,
and the molar teeth are proportionately smaller. The thin plate which
forms the anterior root of the zygomatic arch is deeply emarginated in
front in _R. cuniculoides_ (see Plate 34, fig. 21, _b._); but in _R.
chinchilloides_, the anterior margin of this plate is nearly straight,
(see Plate 34, fig. 20, _c._)
In the form of the lower jaw of the two animals under consideration
there are differences which will be more clearly understood upon
comparing the figures. I will therefore merely notice one remarkable
character which is found in _R. cuniculoides_, and that is, that the
condyloid process is rather deeply concave on the inner side, a
character which does not exist in _R. chinchilloides_, nor do I
recollect having observed it in any other Rodent.
The principal dimensions of the skull of _R. chinchilloides_, are as
follows:—
In. Lines.
Total length 1 2
Width 0 8½
Length of nasal bones 0 6⅓
of incisive foramina 0 4
Distance between the outer surface of the incisors and the
first molar tooth, upper jaw 0 4½
Longitudinal extent of the three molars of the upper jaw,
taken together 0 2¾
Length of a ramus of the lower jaw without the incisor 0 8
_General Observations upon the foregoing Species of Muridæ._
In the foregoing descriptions I have endeavoured to convey an idea of
the characters of the species of mice submitted to me for examination
and description, by Mr. Darwin: there are, however, some points upon
which I have been silent in my descriptions. I allude to the characters
observable in the dentition. I have omitted to notice the various
modifications in the structure of the molar teeth, because I found it
would lengthen the descriptions to no good purpose, inasmuch as of
almost all the species I have made outlines of the molars, which will
convey a more clear idea than any verbal description can do.
Upon an inspection of the Plates, it will be seen, that by far the
greater portion of the teeth figured, may be referred to one particular
type of form or pattern, and that this pattern does not agree with that
observed in the molars of _Mus Rattus_, _M. decumanus_, or _M.
musculus_, whilst these three species agree essentially with each other.
In the young Black Rat (_Mus Rattus_), before the teeth are worn, the
two anterior molar teeth, on either side of the upper jaw, present three
longitudinal rows of tubercles, a central series of larger tubercles,
and on each side of these, a row of smaller ones. The front molar has
three of the larger tubercles arranged along the middle of the tooth;
three smaller ones on the outer side, and two, on the inner side. The
second molars have two central tubercles, two outer, and two inner ones.
The posterior molar is nearly round, the body of the tooth consists of
three principal tubercles, and one small tubercle, situated on the inner
and anterior portion of the tooth.
The corresponding teeth in the young of _Mus bimaculatus_ present a very
different appearance; the molars, instead of having three longitudinal
rows of tubercles, have only two. An idea of the appearance of these
teeth may be formed by removing the inner row of tubercles from the
molars of _Mus rattus_. We should then have, as in _Mus bimaculatus_,
molars of a narrower form, the first tooth presenting six tubercles, the
second, four; and the posterior tooth devoid of the small inner lobe;
the opposing tubercles of each tooth, however, in _M. bimaculatus_, are
of equal size.
The molars of the lower jaw of _Mus bimaculatus_ agree with those of _M.
Rattus_ as to the number of tubercles which they possess; they are,
however, proportionately longer and narrower, and, when a little worn,
these teeth, as well as those of the upper jaw, differ considerably from
those of _M. Rattus_. In the last named animal, when the molars are
slightly worn, the ridges of enamel run completely across the tooth, as
in Figs. 18 and 19, Plate 34. Such is not the case in _M. bimaculatus_
at any age. As soon as the molar teeth are worn, the folds of enamel
penetrate the body of the tooth on each side, and those of one side
alternate with those of the other,—in fact, they very nearly resemble
those of the _Hamsters_ (_Cricetus_).
I have selected the molar teeth of _Mus Rattus_ and _M. bimaculatus_ for
comparison, since I happened to possess specimens displaying both the
young and adult states of each. But had I selected, on the one hand,
almost any of the species brought from South America by Mr. Darwin, and,
on the other hand, the _Mus musculus_ or _M. decumanus_, I should have
had to point out the same distinctions—the former agreeing in dentition
with _M. bimaculatus_, and the latter with _M. Rattus_.
The differences pointed out, between the molar teeth of _Mus Rattus_ and
those of _M. bimaculatus_, I cannot but consider as important, since all
the Old World species of _Mus_ which I have yet had an opportunity of
examining (and they are numerous) agree essentially with the former,
whilst the only _Mus_ from S. America (excepting _M. Musculus_ and _M.
decumanus_, which are carried in ships to all parts of the world) in
which I have as yet found molar teeth like those of _M. Rattus_, is the
_Mus Maurus_, and this it has been stated is possibly a variety of _M.
decumanus_.
Although as yet I have not met with species in the Old World possessing
the characters of the South American _Muridæ_, among those of North
America, several have come under my observation. The _Mus leucopus_,
_Symidon hispidum_, and the species of _Neotoma_ certainly belong to the
same group,[28] as does also the species of the Galapagos Islands,
described in this work under the name _Galapagoensis_.
These considerations have induced me to separate the South American mice
from those of the Old World,—or rather from that group of which _M.
decumanus_ may be regarded as the type,—and to place them, together with
such North American species as agree with them in dentition, in a new
genus bearing the name _Hesperomys_.[29]
Whether this group be confined to the Western hemisphere or not, I will
not venture to say, but I think I may safely affirm that that portion of
the globe is their chief metropolis.
The species of the genus _Hesperomys_, which depart most from the
type—whose dentition is least like figs. 5, _a_, and 5, _b_, Plate 33.,
or 6, _a_, and 6, _b_, of the same Plate—recede still farther from the
genus _Mus_, and approach more nearly (as regards the dentition) to the
_Arvicolidæ_. Among the species here described I may mention as
examples, _M. griseo-flavus_, _M. zanthopygus_, and _M. Darwinii_;—see
the molar teeth figured in Plate 34. figs. 15, 16, and 17,—and among the
North American species, those constituting the genus _Neotoma_. The
latter make by far the nearest approach to the _Arvicolidæ_ of any which
have yet come under my observation, not only in the dentition, but in
the form of the skull and the large size of the coronoid process of the
lower jaw; there is, nevertheless, a tolerably well marked line of
distinction between the crania of the _Arvicolidæ_ and _Neotoma_.
The skulls of the animals belonging to the genera _Castor_, _Ondatra_,
_Arvicola_, _Spalax_, and _Geomys_, which constitute the principal
groups of the family _Arvicolidæ_, when compared with those of the
family _Muridæ_, present, among others, the following distinctive
characters.
The temporal _fossæ_ are always much contracted posteriorly, by the
great anterior and lateral development of the temporal bones; the plane
of the intermolar portion of the palate is below the level of the
anterior portion; the coronoid process of the lower jaw is very large,
the articular portion of the condyloid process is proportionately broad;
the descending ramus, or posterior coronoid process, is so situated that
its upper portion terminates considerably above the level of the crowns
of the molars; this same process is generally[30] directed outwards from
the plane of the horizontal ramus. The incisor teeth of the _Arvicolidæ_
differ from those of the _Muridæ_ in being proportionately broader and
less deep from front to back—they are not laterally compressed as in
_Mus_. The molar teeth are rootless,[31] and the folds of enamel are the
same throughout the whole length of the tooth; whereas in _Mus_ they
enter less and less deeply into the body of the tooth as we recede from
the crown, and towards the base of the visible portion (the tooth being
in its socket) the indentations of the enamel are obliterated.
Now in the species of _Hesperomys_, the molar teeth are always rooted,
and in the form of the skull and the lower jaw they agree with the
_Muridæ_, and do not present the characters above pointed out as
distinguishing the _Arvicolidæ_, and as regards the cranium and lower
jaw, it is only in the genus _Neotoma_ that any approach is evinced.
Of the various groups of the order _Rodentia_ found in South America,
the _Sciuridæ_, so far as I am aware, are chiefly confined to the more
northern parts, and do not occur in the most southern; the _Myoxidæ_,
_Gerboidæ_, and _Arvicolidæ_ are wanting. The species of the family
_Muridæ_ belong to different sections to those of the Old World. Of the
_Leporidæ_ I am acquainted only with one well established species—the
_Lepus Braziliensis_, which however is not found “in tota America
Australi,” as Fischer says, there being no Hare yet found in the more
southern parts, where the _Cavies_ and _Chinchillas_ appear to take
their place. The remaining South American Rodents—certain species of
_Hystricidæ_, the genera, _Echimys_, _Dasyprocta_, _Cælogenys_ and
_Myopotamus_, together with the _Octodontidæ_ and _Chinchillidæ_, all
possess a peculiar form of skull and of the lower jaw, (more or less
approaching to figs. 1, Plate 33, and figs. 23, Plate 34.) which I have
described in the “Magazine of Natural History,” for February 1839, and
which is rarely found in the North American, or Old World Rodents. In
enumerating the above groups, I omitted the _Caviidæ_, because in the
form of the lower jaw they differ somewhat from the rest—they possess,
in fact, a form of lower jaw peculiar to themselves; but in the
Chinchillas[32] the transitions between one form and the other are
found.
The South American _Muridæ_, which form the chief part of Mr. Darwin’s
collection, were none of them procured further north than latitude 30°,
with the exception of those from the Galapagos Archipelago. The species
occur at the following localities.