RATS AND MICE. 273 



is scarcely an instance of a northern species lia\'ing gone further south than the Isthmus of Panama. 

 Southern species have passed to the north of it, but northern seem alwaj's to have stopped on 

 the nortli side of it. 



One other observation occurs to mo, viz. that these coincidences in the limits of the geog'raplucal 

 distribution of the Dipodomys and Geomys furnish an extraneous argument against Professor 

 Brandt's objection to these being classed together, and in support of the view that they are allied 

 to each other, notwithstanding the dissimilarity in their personal appearance. 



MURIXA. 



Rats and Mice. (Maps 78, 79, 80, 82.) This family contains a vast number of species, all 

 bearing much resemblance to each other ; and the discovery of some characters hj which to break 

 it up and render it more manageable is very desirable. Unfortimately, much cannot be expected 

 in this respect : more may be done hereafter by a careful examination of species, and the consequent 

 reduction in their number which will residt. I shall commence with the group which comes most 

 naturally after the external-pouched Rats, viz. — 



The Hamster. (Cricetus.) (Map 78.) The Eats with internal cheek-pouches are not 

 numerous. There are eight or nine sjjecies, the most widely spread of which is the Common 

 Hamster. It ranges westward from the Rhine to the river Obi, and southwards from the Obi 

 and Irtisch to Persia and Caucasus. Other Species extend the range somewhat further east. The 

 C. ruRXJNCULL's and C. Songakus both inhabit the high steppes of Mongolia, and the former is 

 found in the valley of the Onon, north of MongoHa. The genus does not appear, however, to 

 reach the shores of the Pacific. It does not occur in Schrenck's list of the Mammals of Amour- 

 land, nor in Temminck's Fauna of Japan. 



A large grey pouched Rat in West Africa, Cricetomys Gambianus, probably extends over 

 the greater part of Africa, south of the Sahara, as it has been found in Mozambique as well as 

 Senegambia. Two species of another genus, named Sacuostomus by Dr. Peters, have been obtained 

 by him at Mozambique. Some remains referred to species of Hamster have been found in the 

 tertiaries of Sansans and in the marls of Limagne. 



If we now take the remainder of the old genus Mrs, we find that it divides itself very natui-ally 

 Into two sections, — Old-world and New- world, — which are characterized by several characters ; one 

 of which, first pointed out by Watcrhouse, is especially valuable. The Old-world Mice have 

 large and broad molars, each with tubercules placed transversely ; and those in the iqiper jaw have 

 three tubercules in each transverse series. The New- world species have the molars narrower, and 

 only two tubercules in each transverse (or slightly oblique) series. This is a very useful test, as 

 it enables the naturalist to decide at once whether a given species caught on the one or the other 

 continent really belongs to it, or has been introduced from the other. For example, the Black Rat 

 (Mus Rattus) has established itself so completely in North America, that, according to Dr. Giebel, 

 some maintain that America is its native place, and that it has been introduced from thence into 

 Europe. A glance at the upper molars settles this question, and shows that Mi's Rattus is an 

 Old-world species, and that it must have been communicated from it to the New World, instead of 

 being received from thence. In the same way several of the specimens which were obtained by 

 Mr. Darwin from South America, &c., during his voj-age in the " Beagle," and whicli might other- 



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