386 Dr. E. L. Trouessart 07i 



Besperomys, because the subdivisions of this great group of 

 American rats were at that time considered by naturalists 

 merely as subgenera. 



Now the subgenera Rhipidomys, Oryzomys, Calomys, 

 Onychomys, &c. are looked upon as true genera, and several 

 of them are already subdivided. Megalomys having the 

 same importance, it seems right to raise it also to the rank of 

 a genus. 



Yet in 1897, when writing the ' Catalogus Mammalium ' 

 (Pars III. Kodentia), I allowed myself to be influenced by a 

 prior suggestion of Mr. O. Tiioraas *, and in contradiction 

 to the opinion for which I contended, with some reason, 

 in my work of 1881 and 1885, based on the original 

 specimens of Plee (from Martinique) in the Museum of Paris, 

 1 referred, too hastily, Megalomys to the genus Hohchilus, 

 Brandt, as a simple subgenus. Indeed, Megalomys pilorides 

 and Holochilus vuljn'nus are alike only as regards their 

 large size. 



More recently, Dr. C. I. Forsyth Major, in a short pre- 

 liminaiy note f , and without giving tiie reasons for this 

 identification^ coimects Megalomys with the geiius Oryzomys^ 

 another subdivision of the old genus Hesperomys. 



On this occasion I fear my learned friend. Dr. Forsyth 

 Major — -who is, first of all, a paleontologist, that is to say, an 

 anatomist, — let himself be influenced by the cranial features 

 of Mus inlorides, which are, in my opinion, common to all 

 the large species of Muridte, and disregarded the important 

 zoological characters wdiich clearly distinguish Megalomys 

 from Oryzomys. 



In reality Megalomys pilorides and Oryzomys palustris (the 

 type of the last genus) are much more distinct than, for 

 exam])le, Evotomys glareolus and Arvicola amphibius. One 

 is exclusively terrestrial, the other aquatic, and the characters 

 of both are perfectly in accordance with their habits. This 

 will be seen from the following table, in which I quote the 

 words employed by Baird J in his description of Oryzomys 

 j;ahistris, or, more correctly, tliose used by Elliott Coues §, 

 to distinguisli this type species: — 



* "On a Collection of Muiidfe from Central Peru" (Proc. Zool. See. 

 Lond. 1884, pp. 447 et seq.). 



t " The Musk-Rat oi Santa Lucia " (Ann. S: Mag. Nat. Hist. ser. 7, 

 vii. 1901, pp. 204-20G). 



\ ' Mammals of North America,' 1857, pp. 4-58 & 482-483, It should 

 be noticed that the characters of the subgenus Oryzomys (p. 4o8) are 

 incomplete, as only those are mentioned in which it diflfers from Hesper- 

 omys, properly so-called, and from Onychomys. 



§ ' Mouoiiraph of the Rodentia of Nuith America,' Muridae, 1877, 

 pp. 111-11:1 



