694 MR. O. THOMAS ON A NEW [DcC. 14, 



one large genus, with the present genera Holochilus, Hesperomys, 

 Signiodon, Reithrodon, and Ochetodon each representing a subgenus 

 of greater or less extent. Of these names Sigmodon would have to 

 be the one to stand for the genus, antedating Waterhouse's appro- 

 priate name Hesperomys by 14 years. 



As, however, we must for the present consider B. alstoni as a true 

 Reithrodon, the next point we have to consider is its relationship to 

 the other described species. The only species of this rare genus 

 hitherto known are Waterhouse's three original ones, the North- 

 American mice described as belonging to it by Baird and De Saus- 

 sure having been separated by Dr. Coues as a distinct genus under 

 the name of Ochetodon '. At the same time that he founded this 

 last genus. Dr. Coues also divided the true Reithrodon into two 

 subgenera — Reithrodon proper, containing R. typicus and R. citni- 

 citloides, and Euneomys, containing R. chinchilloides only. This 

 subdivision certainly seemed to be justified by Waterhouse's figures 

 and descriptions ; but now that R. alstoni has to be arranged with 

 the others it is at once plain that these subgenera cannot stand as such, 

 that species being, as far as regards the subgeneric characters used 

 by Dr. Coues, precisely intermediate between them. Thus, it has 

 the concave front edge of the zygoma-root, the deep pterygoid 

 fossae, the rounded descending process of the lower jaw, and the 

 short fifth toe of Reithrodon as restricted, and the short incisive 

 foramina, the short palate, and the uncontracted posterior nares of 

 Euneomys. Unless, therefore, we are prepared to make a third 

 subgenus for the reception of this species, a course which I think is 

 hardly necessary, we must abolish the subgenera above named and 

 include all the species under Reithrodon proper. 



With regard to the specific distinctness of R. cuniculoidea and R. 

 typicus, about which Dr. Coues rather naturally expresses some 



Fig. 3. 



Front part of skull of R. typicus. 



doubt, it unfortunately happens that the type of the latter in the 

 British Museum is in an extremely bad condition, having all the 

 cranial and palatal portions of the skull broken away ; but there 

 remains enough to show that the nasal region, though similar, is not 

 identical with that of R. cuniculoides, and that the muzzle is some- 



1 Proc. Ac. Nat. Sci. Phil. 1874, p. 184. 



