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EXPLORATION OF THE CANONS OF THE COLORADO. 
GEOMYS. 
Superior incisors deeply channeled 
along the middle, with or without 
a fine marginal groove. 
Crowns of intermediate molars truly 
elliptical. 
Root of inferior incisor but little pro 
tuberant on outside of base of con- 
dylarramus; end of mandible thus 
only two-pronged, with a knob 
between. 
Zygomata widest across anteriorly, 
thence contracting ; the width be 
hind little, if any, greater than the 
intermastoid diameter of the skull. 
Parietals ridged along their line of 
union with each other. 

Interparietal triangular. 
Nasals approximately parallel-edged 
part way, then suddenly widening. 
Superficies of mastoid bone occupying 
nearly half the occipital surface of 
the skull on each side. 
Bullae ossese less inflated, quite acute 
anteriorly. 
Basi-occipital, in the middle, about as 
broad as the width of the bulla at 
the same point. 
A pair of broad deep pits on the pal 
ate behind, extending forward to 
opposite the penultimate molars. 
THOMOMYS. 
Superior incisors without median sul- 
cus,"but with a fine marginal groove 
(sometimes obsolete). 
Crowns of intermediate molars acute- 
edged exteriorly. 
Root of inferior incisors causing a pro 
tuberance on outside of base of con- 
dylar ramus nearly as high as con- 
dyle itself; end of mandible thus 
singularly three-pronged. 
Zygomata regularly convex outward, 
with a sweeping curve ; their 
breadth across posteriorly deci 
dedly greater than the intermas 
toid diameter of the skull. 
Parietals ridged externally near the 
squamo-parietal suture. 
Interparietal rather pentagonal. 
Nasals widening uniformly from be 
hind forward. 
Superficies of mastoid bone restricted 
to less than a fourth of the occipital 
surface on each side. 
Bullse ossese more inflated, quite ob 
tuse anteriorly. 

Basi-occipital, in tlie middle, much 
narrower than the bulla at the 
same point. 
A pair of slight pits on the palate 
behind, not extending beyond the 
ultimate molars. 
