RATS AND MICE. 275 



Mouse, or less." * The North American species are upwards of a dozen in number. They are 

 generally distributed over the whole country, from the Arctic Circle to Mexico, and it is difficult 

 to say that one region has a greater proportion than another. 



Mr. Salvin's list of Mammals from Guatemala contains three species of HESPEROMTi-s of the 

 North American type, and one of them apparently a species actually found in North America. The 

 South American sub-genus, Calojiys, which comes nearest to the North American, is not appa- 

 rently found further or in greater proportion to the North than any of the other South American 

 8ub-genera. Its metropolis seems to be South Brazil, which, indeed, has furnished a greater 

 number of species of all the sub-genera than any other part of the country. Patagonia and Chili 

 are represented by more than their due jDroportion, in consequence of having been the ground 

 which was worked by Mr. Darwin. Previous to his visit, scarcely any were known to inhabit the 

 south of South America. And notwithstanding that his researches were necessarily very cursory, 

 and intermittent, he added between twenty and thirty species to our lists. 



Reithrodon. The genus Reithrodox was founded by Mr. "Waterhouse, on some Patagonian 

 species of Hesperoivs, of large size, the chief characters being that the incisors are longitudinally 

 grooved while in Hesperomys they are not. They came from the extreme south of that 

 district, and were the sole representatives then known of the genus. Since then, however, some 

 small slender species of Hesperomys, having grooved incisors, have been found in North America, 

 and referred by Leconte and Dr. Baird on the strength of that character, to Reithrodon. Dr. Baird 

 informs us that he has seen neither skulls nor skins of Reithrodon from South America ; but he says 

 that judging from the figures giving by Waterhouse, there are considerable differences, not only 

 in size but in other characteristics. It is, however, imiwssible to indicate these discrepancies without 

 making a careful comjjarison of specimens from the two countries. The South American Reith- 

 RODONS have a body six inches in length, so stout and fidl, and the head so large and much arched, 

 that one species has been called R. cuniculoides (rabbit-like). The tail, also, does not exceed the 

 half of the body. The North American species, on the contrary, are the smallest of the Mice, 

 scarcely more than half the size of the House Mouse, which they otherwise closely resemble in 

 shape and proportion. The taQ is as long as the body alone, or else longer than the head and 

 body together. The shape and character of the skidl are quite different, f 



Until the species of the South American Reithrodon be compared with the North American, it is 

 premature, therefore, to treat them as identical. To do so may lead to misapprehension of their affinities 

 and geographical distribution. I, therefore, in the meantime, speak of the two as the South American 

 Reithrodon and the North American Reithrodon. Three species of the South American genus 

 are kno-mi, one inhabiting the open grassy Savannahs of Maldonado, another the coast of Patagonia, 

 and the third the Straits of Magellan. The North American genus contains four or five species, 

 which are confined, on the Atlantic border, to the Southern States. They are found about St. Louis, 

 and westward to the Rocky Mountains. Species occur also in New Mexico, Sonora, and California. 

 "We have already seen that there are groimds for believing that the break between North and 

 South America, which at some former time must have existed, probably did not occur to the north 

 of Guatemala, but between it and New Granada. The distribution of Hesperomys and of the 

 Reithrodons confirms this view. Besides the three Hesperomys of the northern facies above 

 mentioned, Mr. Tomes' J list of the mammals collected by Mr. Salvin in Guatemala, contains two 



* Baird, op. cit. p. 455. I Tomes in "Proc. Zool. Soc." p. 278, 1861. 



t Baird, op. cit. p. 448. 



