MILLER AND GIDLEY — NEW FOSSIL RODENT 
73 
A NEW FOSSIL RODENT FROM THE OLIGOCENE OF SOUTH 
DAKOTA! 
By Gerrit S. Miller, Jr., and James W. Gidley 
Among some fragmentary jaws of Ischyromys from the Bad Lands of 
South Dakota two species are represented; a large-toothed animal 
resembling Ischyromys typus Leidy, but probably related to Ischyromys 
chrysodon (Cope), and a small-toothed form which may be named and 
described as follows : 
Ischyromys parvidens sp. nov. 
Type. — Imperfect right mandibular ramus containing all four cheekteeth 
(pm 4 in place, moderately worn), No. 9134, U. S. National Museum. Collected 
in the Oreodon Beds of the White River Oligocene, “Bad Lands,” Washington 
Count 3 ^, South Dakota, by J. B. Hatcher. 
Diagnosis. — ^Mandible and teeth in general like those of Ischyromys typus, but 
teeth relatively smaller, the length of the entire toothrow about equal to that of 
three teeth of the larger animal. 
Mandible. — In form the mandible agrees with that of Ischyromys typus. It is 
not thickened as in the species which we suppose to be related to I. chrysodon, 
nor reduced in depth as in Titanotheriomys veterior. Region of masseter attach- 
ment resembling that of I. typus in the position of its anterior border, beneath 
hind margin of mi, and in the relative distinctness of its outline (more clearly 
. defined than in the other large-toothed species). 
Teeth. — Incisor, so far as can be judged from the fragments, robust like that 
of the other members of the genus, not slender as in Titanotheriomys. Its posterior 
termination lies against the inner surface of outer wall of the base of the ascend- 
ing ramus, at alveolar level and separated from posterior border of m 3 by a space 
about equal to the length of this tooth. In I. typus the incisor appears to end 
at the same point in the jaw, but the large size of the cheekteeth brings the 
posterior border of m 3 back nearly to a level with it. Size of cheekteeth both 
actually and relatively much less than in Ischyromys typus and resembling that 
in Titanotheriomys veterior. The length of the toothrow slightly exceeds the 
depth of the mandible at front of mi while in I. typus it is about one and one-half 
times this depth. The enamel pattern shows no important peculiarities. 
Specimens examined. — Five imperfect mandibles. 
Remarks. — The only described member of the family Ischyromyidce 
in which the teeth are as small as those of Ischyromys parvidens is the 
Titanotheriomys veterior of Matthew. In this animal the lower jaw is 
noticeably more slender than in any of the known species of Ischyromys. 
Hence there should be no difficulty in distinguishing between fragmen- 
1 Published here by permission of the Secretary of the Smithsonian Institution. 
