MAMMALIA. 



75 



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 alter- 

 nate 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 com- 

 parison, 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 31. decumanus, I should have had to point out the same 

 distinctions — the former agreeing in dentition with 31. bimaculatus, and the latter 

 with 31. Rattus. 



The differences pointed out, between the molar teeth of Mus Rattus and 

 those of 31. 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 3Ius 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 Muridce, 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,* as does also the 

 species of the Galapago Islands, described in this work under the name Gala- 

 pagoensis. 



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 Hesperamys.-\ 



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 



* I am acquainted with seven North American Species of Muridce, all of which possess the dentition of Hes- 

 peromys. 



t 'Etrwepos, West, and Mus. 



