2. Reithrodon typicus.
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.
WEST COAST OF SOUTH AMERICA.
Galapagos Archipelago.
Mus Jacobiæ.
—— Galapagoensis.
Coquimbo.
Mus longipilis.
—— Renggeri.
—— Darwinii.
Valparaiso.
Mus Renggeri.
—— decumanus.
Concepcion.
Mus longicaudatus.
Chiloe and Chonos Archipelago.
Mus brachiotis.
EAST COAST OF SOUTH AMERICA.
Maldonado.
Mus decumanus.
—— maurus.
—— Musculus.
—— tumidus.
—— nasutus.
—— obscurus.
—— arenicola.
—— bimaculatus.
—— flavescens.
Reithrodon typicus.
Buenos Ayres.
—— Mus decumanus.
Bahia Blanca.
Mus Braziliensis.
—— elegans.
—— gracilipes.
Rio Negro.
Mus griseo-flavus.
Port Desire.
Mus canescens.
St. Julian.
Reithrodon cuniculoïdes.
—— xanthopygus.
Reithrodon cuniculoïdes.
Santa Cruz.
Mus canescens.
—— micropus.
—— xanthopygus.
Reithrodon cuniculoïdes.
Falkland Islands.
Mus decumanus.
—— Musculus.
Straits of Magellan.
Mus xanthorhinus.
—— Magellanicus.
Reithrodon chinchilloïdes.
Section—HYSTRICINA.
Family— ——?
Myopotamus Coypus.
Myopotamus Coypus, Auct.
“This animal, in Chile, is known by the name of “Coypu;” at Buenos Ayres, where an extensive trade is carried on with their skins, they are improperly called ‘nutrias,’ or otters. In Paraguay, according to Azara, their Indian name is ‘guiya.’ On the east side of the continent they range from Lat. 24° (Azara) to the Rio Chupat in 43° 20′;—distance of 1160 miles. This latter river is 170 miles south of the Rio Negro, and the intervening space consists of level, extremely arid, and almost desert plains, with no water, or at most one or two small wells. As the Coypu is supposed never to leave the banks of the rivers, and being, from its web-feet and general form of body, badly adapted for travelling on land, its occurrence in this river is a case, like so many others in the geographical distribution of animals, of very difficult explanation. The same remark is indeed applicable, but with less force, to its existence in the Rio Negro. On the west coast, it is found from the valleys of central Chile (Lat. 33) to 48° S., or perhaps even somewhat farther, but not in Tierra del Fuego. So that, on the Atlantic side of the continent, the plains of Patagonia check its range southward, as, on the Pacific side, the deserts of Chile do to the north. Its range, including both sides, is from 24° to 48°, or 1440 miles. In the Chonos Archipelago these animals, instead of inhabiting fresh water, live exclusively in the bays and channels which extend between the innumerable small islets of that group. They make their burrows within the forest, a little way above the rocky beaches. I believe it is far from being a common occurrence, that the same species of any animal should haunt indifferently fresh water, and that of the open sea. We shall see that the Capybara is sometimes found on the islands near the mouth of the Plata; but these cannot be considered as their habitual station in the same manner as the channels in the Chonos Archipelago are to the Coypu. The inhabitants of Chiloe, who sometimes visit this Archipelago for the purpose of fishing, state that these animals do not live solely on vegetable matter, as is the case with those inhabiting rivers, but that they sometimes eat shell-fish. The Coypu is said to be a bold animal, and to fight fiercely with the dogs employed in chasing it. Its flesh when cooked is white and good to eat. An old female procured (January) amongst these islands, weighed between ten and eleven pounds.” D.