SACCOMYIDAi—PEROGNATHIDIN AA—PEROGNATHUS. 499 
on the interior side; the back upper molar is simply single-rooted. The 
under molars have each a pair of roots, aligned lengthwise in a single series, 
but the two roots of the back lower molar are imperfectly distinguished. 
With these last exceptions, each root of all the teeth has its own distinct 
socket in the alveolus. ; 
In the perfectly unworn state, the crowns of the molars are studded with 
tubercles in regular transverse series. In the upper jaw, the anterior molar 
has four,—an anterior, a posterior, an exterior, and an interior, with perhaps 
another one part way up the anterior lobe. The second and third upper 
molars have each six tubercles, in two straight transverse rows of three each, 
these rows separated by a deep sulcus. The smaller cir- 
cular back upper molar tends indistinctly to a similar state. 
In the lower jaw, the tuberculation is very similar; but 
the four tubercles of the first molar are in an anterior and 
posterior pair, and on the last one the tubercles become 
indistinct. The teeth present a very different aspect when 
the tubercles are ground off with wear. Each transverse 
row of tubercles becomes converted into an island of 
dentine, there being thus, on the intermediate molars at 
any rate, a pair of such transverse dentine islands separated 
hy a double ridge of enamel partition, between which is 
the bottom of the sulcus already mentioned. This enamel 
fold makes in from the outer side of the tooth uearly 
to the inner side. The front molar shows a little isolated 
island of dentine anteriorly, nearly circular, and a broad 
transverse one posteriorly. The state of the under teeth 
is substantially the same. RalanGlont ar fae 
The upper incisors are small, compressed, with a Left ear, in each case, 
Ae : twice the natural size: 
strong backward set. Their face is deeply channelled upper fig. P. monticota; 
middle fig. P. penicilla- 
tus ; lower fig. Cricetodi- 
beted down so that the groove is visible laterally as well pus favus. 
with a longitudinal groove, and the exterior moiety is rab- 
as from the front. This is a prominent character (shared by Cricetodipus and 
Dipodomys) in distinction from Heteromys. The under incisors are small 
and simple. : 
Before leaving this portion of the subject, I may as well mention a curi- 
ous circumstance: the ease with which the skulls of Perognathus and Crice- 
