472 MONOGRAPHS OF NORTH AMERICAN RODENTIA. 



palate extends back of the molar scries, and the contracted interpterygoid . 



space is narrowly angular. The maxillo-palatinc suture of Zapus, likewise, 

 is differently located, being opposite the interspace between the penultimate 

 and preceding molar, instead of much farther back. There is a pair of con- 

 spicuous palatal foramina opposite the penultimate molar. The contour of 

 the palate differs from that of Mus, and perhaps a majority of allied Rodents, 

 in being broader in front than behind. The incisive foramina are of great 

 length, as well as quite broad, reaching from little behind the incisors to oppo- 

 site the molars; the perforation is half in the intermaxillary, half in the max- 

 illary; the bony septum is swollen except at its posterior part. 



The form of the descending process of the mandible is a strong charac- 

 ter of Zapus in comparison with Mus, &c, in which this plate of bone is 

 more or less squarish, and vertical or nearly so. In Zapus, the same plate is 

 strongly twisted out of the axis of the jaw, standing diagonally outward and 

 upward, — very much, in fact, as in Saccomyidte. The coronoid is rather weak, 

 falcate, acute, with a strong slope ; it slightly overtops the condyle. The 

 latter sets strongly backward, though it is rather more erect than in Mus. The 

 incisor causes a moderate protuberance outside, at the root of the condylar 

 process. Inside, nearly opposite, is the conspicuous foramen of the inferior 

 maxillary nerve. 



B. Dental characters. — The superior incisors are short and stout, with a 

 strong curve; their anterior faces strongly sulcate, with the outer half of the 

 tooth rabbeted down so that the groove is plainly visible from the side. The 

 inferior incisors are not specially noteworthy. The molar series differs from 

 that in Muriilc proper (except Smintkus) in the presence of a small premolar 

 in the upper jaw, with no tooth to correspond in the lower series. This 

 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 upper premolar is the 

 smallest of the whole, and simply circular; the next two are about equal in 

 size; the last is much smaller A similar proportion is seen in the under 

 series. The pattern of the molar crowns is much complicated. 



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

 development of the hind limbs (much as in Dipodidcs or some forms of Sac- 

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



