74 
JOUKNAL OF MAMMALOGY 
tary mandibles. While the material which we have examined is not 
sufficient to form the basis of a revision of the group it indicates the 
existence of at least four species as shown in the following key: 
8 5 
Length of first three molariform teeth about ^ 
8.5 
Depth of mandible at middle of m 2 , about 8. . . 
Depth of mandible at middle of m 2 , about 10 
Length of first three molariform teeth about 
10.0 
10.0 
Titanotheriomys veterior 
, . .Ischyromys parvidens 
Area of muscle attachment in front of antero-external root of pm^ a well 
defined, rounded pit Ischyromys chrysodon 
Area of muscle attachment in front of antero-external root of pm** an ill 
defined shallow depression Ischyromys typus 
The form described by Cope as Colotaxis cristatus appears to have 
been based on jaws of Ischyromys typus with the narrow-crowned milk 
premolar in place. We have not seen jaws of true Ischyromys chrysodon 
from Colorado, but those of the nearly related and perhaps identical 
species occurring in the Bad Lands differ from those of Ischyromys 
typus in their obviously greater breadth; about 7-8 mm. instead of 
about 5.5 mm. at level of posterior cheektooth. 
