472 MONOGRAPHS OF NORTH AMERICAN RODENTIA. 
palate extends back of the molar series, and the contracted interpterygoid 
space is narrowly angular. The maxillo-palatine 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 Rodeuts, 
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 Zupus, the same plate is 
strongly twisted out of the axis of the jaw, standing diagonally outward and 
upward,—very much, in fact, as in Saccomyide. 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. i 
B. Dental characters—The superior incisors are short and stout, with a 
strong curve; their anterior faces strongly suleate, 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 Muridz proper (except Sminthus) 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 Dipodide or some forms of Sac- 
comyide), and especially of the pes itself; an unusual length of tail, which 
