100 MR. O. THOMAS ON RODENTS FROM PERU. [Jan. 17, 
peromys now usually admitted. Of these Calomys claims four species, 
one being new ; Rhipidomys also four, of which two are new ; and 
Habrothrizx the remaining two, both of which are previously-known 
species. There are thus three new species in the collection; and of 
two of the others I am somewhat doubtful of the determination ; so 
that I think it possible that either or both of them will in the end 
turn out to be really different from the species to which I have pro- 
visionally referred them. 
The chief previous information bearing on this subject is comprised 
in Tschudi’s classical work on the fauna of Peru’, and in Mr. Tomes’s 
papers on the Mammals collected in Ecuador by Mr. Fraser*. 
Of the 6 Muride mentioned by Tschudi, only one, Hesperomys 
leucodactylus, was found by M. Stolzmann ; while of the 13 brought 
from Ecuador by Mr. Fraser he obtained six, or just about half; so 
that from his well-preserved spirit-specimens I have been able to 
supplement the descriptions given by Mr. Tomes, many of which 
were drawn up only from skins. 7 
It is perhaps well to mention that, when describing these Rats and 
Mice, I found it possible, owing to their excellent state of preservation, 
entirely to dry the hairs, so that the colour and texture of the fur, 
and the general appearance of the animals were just as they 
would have been if the specimen had been examined when recently 
killed. I have been unfortunately unable to supplement from this 
collection the notes recently published by me with regard to the 
comparative lengths of the different parts of the alimentary canal’, 
because the intestines had been removed from all the specimens 
before they came into my hands. 
Of the new species obtained by Mons. Stolamann, H. spinosus* is 
perhaps the most interesting, as being the first Hesperomys that has 
been found with spiny fur. I have long expected that such a form 
would be discovered. There are so many examples of tropical 
species of the neighbouring genus Mus which possess spines in their 
fur, that I have always been surprised at there being no spiny 
members of such a large and variable tropical genus as Hesperomys. 
The present discovery of a spine-clad Vesper-mouse is therefore pro- 
porticnately interesting. 
As in the Old-world Mus *, so here in Hesperomys, I find that 
the number of mamme is both very constant and very distinctive of 
1 Pp, 177-184 (1844) 
2 P.Z.S. 1858, p. 546; 1860, pp. 211 & 260. 
3 P. Z.8. 1880, p. 696. 
* Infra, p. 105. 
® See P. Z.8. 1881, p. 531, ke. 
§ The variation in the number in some of the common species of Mus, viz. in 
M. decumanus, rattus, and alexandrinus, has caused this character to fall into 
clisrepute among writers on Rodentia; but, with the exception of these species, and 
of one or two others which have the unusually large number of from 14 to 18 
mamma, I have never, in any single instance, found a specimen of either Mus 
or Hesperonys in which the number differed from that normal to the species. 
I do not of course assert that individual variations do not occur, but only that 
they must be extremely rare, as I have never met with any in all the large series 
of specimens that I have examined with special reference to this point. 
