258 



that in Miiridw proper in the presence of a small anterior molar (pre- 

 molar) ia the upper jaw, with no tooth to correspond in the lower series. 

 The minute premolar is single-rooted ; the three following teeth have 

 three roots apiece— a lengthwise pair of slender fangs outside, and a 

 single stout fang, apparently formed of two coalesced roots, inside. The 

 lower molars have each a pair of roots, in single lengthwise series. The 

 anterior upper molar is the smallest of the whole, and simply circular; 

 the next two are about equal in size; the last is much smaller. A simi- 

 lar proportion is seen in the under series. The pattern of the molar 

 crowns is much complicated. 



External characters. — A general murine form is modified by the great 

 development of the hind limbs (much as in Bipodidcc or some forms of 

 Saccomyidce), and especially of the pes itself; an unusual length of tail, 

 which greatly exceeds that of the body; a peculiar condition of the 

 external ear ; and a i)hysiognomy quite like that of the Saccomyidw. 

 There are also well- developed internal cheek-pouches, shared in a less 

 degree, however, by various American Muridce.* These pouches, as 

 well as can be judged from alcoholic specimens, are relatively about as 

 large as those of Tamias for Instance. 



The bod^' of Zapus is large behind, in correlation with the greatly-de- 

 veloped posterior limbs, and tapers to the lore in a regular manner ; the 

 head being comparatively small, and there being no noticeable constric- 

 tion of the neck. The head is conoidal, with a prominent and rather 

 blunt snout and retreating under jaw. The rather small eye is mid- 

 way between the nose and ear. The upper lip is not visibly cleft, and is 

 densely hirsute, with a fringe of hairs depending over and almost hiding 

 the small front teeth. The naked muffle is of rather small size, and en- 

 tirely inferior in position; above it, the hairy skin crosses with a deep 

 transverse crease, forming a sort of imperfect overhanging flap, which is 

 freely movable back and forth, even in alcoholic specimens, and looks as 

 If it might be drawn down to partially cover the nostrils. ([ have ob- 

 served much the same thing in Saccomyldw.) The nose-pad is impressed 

 with a pair of median vertical grooves, and a transverse one is seen in 

 some cases. The nostrils are completely lateral in position. The whis- 

 kers are rather sparse, but some of them are nearly half as long as 

 the body. 



The sti-ucture of the external ear is rather remarkable (among rodents) 

 for the provision for perfect closure of the meatus, as in the Soricidce for 

 instance. The antitragus develops into a great flap, completely rever- 

 sible, and capable of being applied against the meatus ; and such, in fact, 

 appears to be its usual position. The tragus, likewise, expands into a 

 wide frill, or thin, free, rounded border, which ordinarily lies in apposi- 

 tion to the antitragal lobe opposite, completing the closure of the ear. 

 On turning over these two flaps, the vestibule of the ear is seen to be of 

 unusually large dimensions. The conch itself is of an ordinary contour, 

 coming to a blunt point above ; the anterior third is folded close back. 

 The back of the ear and the fold of the conch are sparsely pilous; the 



*The presence of cheek-pouches in the genus Hesperomys was first noted iu 1830 by 

 Gapper, who referred specimens of the common Hesperomys leucopus to Cricetiis on this 

 account, establishing a species C. myoides. In this matter, he was succeeded by Baird 

 in 1857, who also recognized the pouches, aud endorsed a Resperomys myoides mainly 

 upon this feature, failing, however, to observe that they also existed in other species 

 of the same genus. At the same time that Mr. J. A. Allen announced the before 

 unknown pouches of Zapus hudsonius, he also showed that they occurred as well in va- 

 rious species of Hesperomys ; and my subsequent dissections have satisfied me that 

 pouches are i^resent in all the North American species of Hesperomys proper; i. e., the 

 subgenus Vesperbmis as established by me ; Proc. Acad. Nat. Sci. Phila., 1874, 178. 



