ZAPODID^— ZAPUS HUDSONIUS— EXTERNAL CHARACTERS. 473 



greatly exceeds that of the hod} 7 ; a peculiar condition of the external ear; 

 and a physiognomy quite like that of the Saccomyidce. There are also well- 

 developed internal cheek-pouches, shared, in a less degree however, by various 

 American MuridtB.* These pouches, as well as can be judged from alcoholic 

 specimens, are relatively about as large as those of Tamias for instance. 



The body of Zapus is large behind, in correlation with the greatly-devel- 

 oped posterior limbs, and tapers to the fore in a regular maimer; the head 

 being comparatively small, and there being no noticeable constriction of the 

 neck. The head is conoidal, with a prominent and rather blunt snout and 

 retreating under jaw. The rather small eye is midway 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 entirely 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 nos- 

 trils. (I have observed much the same thing in Saccomyidce.) 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 

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

 body. 



The structure 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 reversible, 

 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 apposition with the anti- 

 tragal 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 



* The presence of cheek-pouches in the genus Hesperomys was tirst noted in 1830 by Gapper, who 

 referred specimens of the common Hesperomys leucopus to Cricetus on this account, establishing a species 

 C. myoides. In this matter, he was succeeded by Baird in 1857, who also recognized the pouches, and 

 endorsed a Hesperomys myoides mainly upon this feature, failing, however, to observe that they also 

 existed iu 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 iu various species of 

 Hesperomys ; and my subsequent dissections have satisfied me that pouches are present in all the North 

 American species of Hesperomys proper ; i. e., the subgenus Vesperimus as established by me : Proc. Acad. 

 Nat. Sci. Phila. 1374, 178. 



