142 MEMOIRS OF THE CARNEGIE MUSEUM 
folding of the enamel usually seen in this genus. In S. fossor the folding of the 
enamel is somewhat similar in position, although not so deep as that in S. montanus. 
In S. fossor the tooth has a slightly greater antero-posterior diameter than m,. It 
has rounded anterior and posterior faces. 
Molar 1.— With exception of the less rounded anterior face, and the deeper and 
more oblique external enamel-folding, this tooth is like the one in advance of it. 
In width and length the tooth has the same diameter. 
Molar 2.—'This tooth is similar to My with a slightly less transvere diameter. 
On the tooth in the left ramus the posterior fossette is divided in two portions, 
while that of the opposite tooth is undivided. 
Molar 3. —'The characters of the third molar are essentially those of the preced- 
ing molars 1 and 2. The tooth is smaller than those in advance of it. The teeth 
are quite prismatic, and terminate in short peg-like roots. 
THE Skuut. Plate XVII., figs. 1, 2, 3. 
The skull of S. fossor is comparatively large, with a rather low occiput, a low 
sagittal crest, moderately long muzzle, and broad zygomatic arches. In general 
outline the skull has a closer resemblance to that of Aplodontia rufa* than to any 
other recent rodent. In studying the skull in detail there are many striking dif- 
ferences between this and the recent genera. The nasals extend comparatively 
farther back than in Aplodontia or Cynomys, so that the fronto-premaxillary suture 
is more sinuous across the face and top of the skull than in these genera. Anteriorly 
the nasals terminate rather bluntly, not overhanging the premaxillaries. The lateral 
border of the nasal is supported entirely by the premaxillary, as in Aplodontia. 
They are broad and flat anteriorly, forming the roof of a triangular anterior narial 
opening. 
The frontals are short, broad anteriorly, with a greatly constricted interorbital 
region. ‘They terminate posteriorly in a wedge-shaped connection with the parietals. 
The supraorbital rugosity is quite heavy in some individuals of this species, and the 
temporal ridges take their origin at this swelling and rapidly unite opposite the in- 
terorbital constriction. There is no postorbital process on the frontal. In this 
respect the fossil is similar to Aplodontia. 
In S. fossor the postorbital area of the frontal is more rounded than in Aplodontia, 
and the postorbital constriction is greater. There is a distinct sagittal crest in the 
ty pe. 
1 The generic name first proposed in 1829 by Richardson was Aplodontia, and, while not satisfactory to a purist in 
Greek etymology, has priority, and must stand.—Zditor. 
