SACCOMYIDA—DIPODOMYIN 4!--DIPODOMYS. 585 
sponding measurement of the hind limb, that is, from the knee outward, is 
three-fifths or more of the whole length, and the hind foot alone is about one- 
third of such dimension. There is no noticeable bulkiness of the haunches, 
but the hams are massive, flattened-conoidal, suddenly contracting at the 
lower third, where the crus is of much less calibre than the foot. There is 
a prominent heel, and an obvious tendo Achillis, The metatarsus is subeylin- 
drical, widening to.a broad foot with strong digits. The whole foot reminds 
ove of a rabbit’s; a resemblance heightened by the dense furring of the whole 
sole, excepting a small calcaneal tuberosity. There are four perfect digits, 
of which the 3d is the longest, the 4th and 2d are successively a little shorter, 
and the 5th is much shorter still; while the 1st is a mere rudiment, entirely 
removed from the rest, nearly half-way up the foot. This bears a stumpy 
claw; the other claws, though small, are well formed, stout at base, com- 
pressed, little curved, and acute. 
The tail exceeds the head and body in length, even without the penicil- 
late tuft of hairs at the end, which projects an inch or more beyond the ver- 
tebree. It is of somewhat quadrangular shape, the flattening being especially 
appreciable on the under side, and is of nearly uniform calibre throughout, 
spriaging directly from the body without any basal enlargement, and but little 
taper at the end. It is closely and uniformly clothed with rather stiffish hairs 
for most of its length, the hairs gradually lengthening on the further half into 
the well-formed terminal brush. 
The pelage differs from that of the allied genus Perognathus in its solt- 
ness, length, and fineness, and has a silky gloss in life. It also lies with 
extreme smoothness on the colored areas of the body; the hairs are plumbe- 
ous basally, as usual; on the white parts they are unicolor to the roots. 
In the male, during the rutting season, there is an immense perineal 
enlargement, strictly circumscribed from neighboring parts, due to the turgid- 
ity of the virile organs within. The anus presents in the centre of this 
enlargement with the sheath of the penis justin front, quite prominent. 
There is a comparatively large os penis. In the female, the anal and genital 
orifices are in still closer relation; the ostium vaginze is prolonged in front 
into a dependent lobe, occupying the site of the penial sheath of the male. 
The pattern of coloration of Dipodomys is peculiar and diagnostic of the 
genus—the striped tail and white band across the hips are not found else- 
where. All the upper parts are fulvous or tawny-brown, closely lined with 
