Red-backed Voles of Japan — JAMESON 
601 
instead of four pairs. The form of the skull 
resembles that of C. rutilus and the other species 
of Clethrionomys s. str. (not including rufoca- 
nus, andersoni, imaizumii, and niigatae ) : the 
skull is delicate and rounded and the postorbital 
projections are barely developed. The enamel 
pattern tends to be rounded rather than angular; 
Thomas noted that the closed triangles tend to 
be little broader than long ( Fig. 1 ) . Phaulomys 
stands apart from Clethrionomys because the 
molars are rootless in the adult. Thomas also 
pointed out the encapsulated root of the second 
upper molar which projects into the orbital 
fossa; this invariably occurs in microtine rodents 
when the molars are not rooted and is simply 
another way of stating this condition. Since the 
description of smithi, many hundred of speci- 
mens have been collected and the molars are 
known to be rootless even in old adults. (Hin- 
ton’s [1926} observation to the contrary re- 
sulted from his confusing Clethrionomys rufo- 
canus bedfordiae and perhaps C. rutilus mikado 
with smithi .) Recent studies by Japanese 
workers (e.g., Tokuda, 4955, and Imaizumi, 
1957 ) indicated that smithi is allied to the Chi- 
nese species of Anteliomys (a synonym or sub- 
genus of Eothenomys ) , and the proper name for 
this species is Eothenomys smithi. Nineteen 
specimens were examined. 
Eothenomys kageus Imaizumi, 1957 
Eothenomys kageus Imaizumi, 1957, Bulletin, 
National Science Museum (Tokyo), vol. 
3, no. 3, p. 204. (Holotype from Yamura- 
machi, Minamitsuru-gun, Yamanashi-ken, 
Honshu. ) 
Imaizumi ( 1957 ) named the form from the 
northeastern part of Honshu as E. kageus. From 
the detailed description of kageus, it is apparent 
that kageus and smithi are very close. E. kageus 
has two pairs of mammae whereas in smithi 
there are three pairs. The size, color, and body 
measurements of E. kageus are very close to 
those of E. smithi. The enamel patterns of the 
two species are also very similar (Fig. 1 ) . The 
original description of E. kageus included a dif- 
ference in the posterior angle of the zygomatic 
arch; but the explanatory drawing (Imaizumi, 
1957: fig. 3) indicated that the angle measured 
on E. kageus was different from that taken from 
E. smithi, and the data are not comparable. The 
differences in the bacula of E. kageus and E. 
smithi may be due to individual variation: spe- 
cimens of both species are extremely variable 
and there seem to be no constant differences in 
the bacula of the two named forms (Fig. 2). 
Six specimens were examined. Future studies 
may reveal that E. kageus is a subspecies of E. 
smithi. 
Eothenomys smithi and E. kageus are similar 
in habits and habitat. They dwell in forested re- 
gions from sea level to elevations of 2500 m. 
or more, but generally do not occur together 
with Clethrionomys niigatae, imaizumii, or an- 
dersoni. On Yatsugatake, E. kageus occurs up to 
about 2000 m., above which level C. niigatae is 
found; but on the eastern slope of Ontake, 
where C. niigatae does not occur, E. smithi ex- 
tends well above 2000 m. (Tokuda, 1950); 
and on Mt. Fuji (where C. niigatae is absent) 
E. kageus is found to the upper limit of the 
forest (Imaizumi, 1944). On Kyushu, E. smithi 
is the only red-backed vole, and it lives in 
wooded areas from at or near sea level to the 
highest peaks, becoming more abundant at the 
higher elevations. On Shikoku, where Microtus 
montebelloi is not found, E. smithi may move 
from the forest to relatively open grassy or 
brushy areas (Ota and Jameson, in press). 
Eothenomys does not occur on Hokkaido. 
The bacula of the red-backed voles in Japan 
( Fig. 2 ) are rather variable within a given spe- 
cies, and provide rather poor taxonomic char- 
acters at the specific level. Some of the smaller 
bacula are obviously from young animals but 
even examples of the same size may be remark- 
ably dissimilar. There are differences in both 
size and shape of the shaft and its base as well 
as in the three prongs. There seem to be no 
distinctions between the bacula of Eothenomys 
kageus and E. smithi unless the shaft in kageus 
is slightly more slender. There are probably no 
differences between the bacula of Clethrionomys 
niigatae and C. rufocanus bedfordiae, but C. 
imaizumii has relatively larger prongs which are 
rather divergent, and in this respect resembles 
those of some species of Microtus. Most bacula 
of these voles have a slight keel on the median 
tyne, a character of the bacula of both Clethri- 
onomys and Microtus. No bacula of C. ander- 
soni were available. 
