320 PROCEEDINGS OE THE ACADEMY OF 



hams are massive, flattened-conoidal, suddenly contracting at the 

 lower third, where the cms is of much less calibre than the foot. 

 There is a prominent heel, and obvious tendo Achillis. The 

 metatarsus is subcylindrical, widening to a broad foot with strong 

 dio-its. The whole foot reminds one 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, compressed, little curved, and acute. 

 The tail exceeds the head and body in length, even without the 

 penicillate tuft of hairs at the end, which projects an inch or more 

 beyond the vertebrae. It is of somewhat quadrangular shape, the 

 flattening being especially appreciable on the under side, and is of 

 nearly uniform calibre throughout, springing 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 softness, length, and fineness, and has a silky gloss in life. It 

 also lies witli extreme smoothness on the colored areas of the 

 body ; the hairs are plumbeous basally, as usual ; on the white 

 parts they are unicolqr to the roots. 



In the male, during the rutting season, there is an immense 

 perineal enlargement, strictly circumscribed from neighboring 

 parts, due to the turgidity of the virile organs within. The anus 

 presents in the centre of this enlargement with the sheath of the 

 penis just in 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 vaginas 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 diag- 

 nostic of the genus the striped tail and white band across the 

 hips are not found elsewhere. All the upper parts are fulvous or 

 tawny brown, closely lined with fuscous to a greater or less 

 extent and intensity. Some specimens, with the least dusky, are 



