Family—MURIDÆ.
1. Mus decumanus.
Mus decumanus, Auctorum.
In the extensive collection of Rodent animals brought home by Mr. Darwin, I find several specimens of the above named species, that is to say, animals which resemble the European specimens of Mus Decumanus in all those characters which are the least liable to variation in individuals of the same species, such as the proportions which the various parts of the animal bear to each other: they differ, however, somewhat in colouring.
Buenos Ayres, Maldonado, Valparaiso, East Falkland Island, and Keeling Island, are each, it appears, infested with the common European rat. I have now before me two specimens from East Falkland Island, and one specimen from each of the other localities, and among these I find none equal in size to the largest European specimens: as regards the colouring, the Buenos Ayres specimen differs only from the English specimens of Mus Decumanus, in having the upper parts of a richer and deeper hue, owing to the tips of the shorter hairs being of a deep yellow instead of pale yellow, and in having a rusty tint over the haunches.
Mr. Darwin found this variety “common about houses in the country around Buenos Ayres.”
In the Maldonado variety, the shorter hairs of the upper parts of the body are of a rusty yellow colour at the apex, in other respects it resembles the British variety. The rusty yellow colour of the tips of the hairs produces a general reddish hue, which is the more conspicuous, when the animal is placed near an English specimen. “Was caught in a house, at Maldonado. I saw a specimen of the common gray English, or Norway rat, lying dead in the streets, and it certainly had a very different appearance from these red rats. The latter, I saw crawling about the hedges in the interior provinces at Santa Fé, and likewise in the forest of the island of Chiloe. This latter fact, however, is a strong argument against its being aboriginal, since I did not find even one undoubted American species, out of the many which I collected, inhabiting both sides of the Cordillera.”—D.
The specimen from Valparaiso very closely resembles that from Maldonado; it is, perhaps, a little less red. “Common about the houses in the town of Valparaiso.”
The two specimens from East Falkland are of a brighter hue, and have less gray in their colouring, than in the European variety of the common rat. “One of them was caught in a Bay, which is sometimes frequented by shipping, but which is distant thirty or forty miles from any habitation. These rats have spread, not only over the whole of East and West Falkland, but even on some of the outlying islets. When the cold, wet, and gloomy nature of the climate is considered, it is surprising that these animals should be able to find food to live on.”—D.
The general hue of the Keeling Island specimen, is deep brown, the longer hairs of the upper parts of the body being, as usual, black; but the shorter hairs, instead of having the pale yellow tint which we observe in the European, (or, rather, British) specimens of Mus Decumanus, are of a deep, rusty yellow. The most remarkable difference, however, consists in the colouring of the under parts being of a yellowish tint, and, towards the root of the tail, of a very distinct buff yellow: the feet are brownish.
“This rat is exceedingly numerous on some of the low coral islets forming the margin of the Lagoon of Keeling Island, in the Indian Ocean. The climate is dry and hot. The rats are known to have come in a vessel from the Mauritius, which was wrecked on one of the islets, which is now called Rat Island. They appeared stunted in their growth, and many of them were mangy. They are supposed to live chiefly on cocoa-nuts, and any animal matter the sea may chance to throw up. They have not any fresh water; but the milk of the cocoa-nut would supply its place.”—D.
The principal dimensions of the above animals are as follows:—
| Specimen from Buenos Ayres | Maldonado. | Valparaiso. | East Falkland. | East Falkland. | Keeling Island. | ||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| In. | Lines. | In. | Lines. | In. | Lines. | In. | Lines. | In. | Lines. | In. | Lines. | ||
| Length | from nose to root of tail | 9 | 9 | 9 | 3 | 8 | 6 | 8 | 9 | 9 | 0 | 8 | 3 |
| of tail | Imperfect | 6 | 0 | 6 | 6 | Imperfect | 6 | 0 | 6 | 6 | |||
| of tarsus | 1 | 7 | 1 | 7 | 1 | 7 | 1 | 7 | 1 | 7 | 1 | 7 | |
Upon comparing the skull of the Valparaiso variety with that of a British specimen of Mus decumanus, I could perceive no difference. A skull from West Falkland did not differ, neither did the dentition of the Keeling Island specimen above noticed. A perfect specimen of this last I have not had an opportunity of examining.
2. Mus (DECUMANUS var. ?) MAURUS.
Mus maurus, Waterh. in Proceedings of the Zoological Society of London, for February, 1837, p. 20.
M. pilis suprà purpurescenti-nigris; subtùs plumbeis; auribus parvulis, pallidè fuscis: caudâ corpus ferè æquante.
Description.—The character of the fur of this animal nearly resembles that of Mus decumanus; it is, however, of a harsher nature: the general colour of the upper parts and sides of the body is purple-black, arising from the longest hairs being of this colour, and likewise the tips of those which are next in length; the latter, however, excepting at the tip, are white, and this white is not entirely hidden, even when the hairs are in their ordinary position: on the head the hairs assume a brownish hue, and are tolerably uniform: the limbs, and under parts of the body, are of a deep gray colour, with a faint purple-brown wash: the under fur is gray: the ears are small, of a brown-white, or very pale brown colour, and furnished with minute brown hairs: the small, scattered, bristly hairs of the tail are of an uniform brownish black colour. The hairs of the moustaches are black at the base, and grayish at the apex.
| In. | Lines. | ||
|---|---|---|---|
| Length | from nose to root of tail | 11 | 3 |
| of tail | 7 | 6 | |
| of tarsus | 1 | 8 | |
| of ear | 0 | 6¼ | |
| from nose to ear | 2 | 2 |
Habitat, Maldonado, La Plata, (June.)
This rat is very closely allied to Mus decumanus, and I think may possibly prove an extraordinary local variety of that animal. Having but one skin, and no skull, I am unable to satisfy myself on this point. Its size, as will be seen by the admeasurements, exceeds that of the common rat, or, rather, it exceeds ordinary specimens of that animal, for I have seen some which were equal to it.
“It was killed near Maldonado, where it frequented holes in the sand hillocks near the shore. It is likewise found on the island of Guritti. If ships are ever infested with these monstrous rats, the above-mentioned localities are very likely places to have received colonies by such means. An old male weighed fifteen ounces and three quarters. The ears of this rat, when alive, were of a pale colour, which made a singular contrast with the black fur of its body.”—D.