54 THE DESEADO FORMATION OF PATAGONIA 
While agreeing in the above general features, there is 
great variation among the various forms. The first upper 
and lower incisor may be greatly enlarged or of normal 
size. There is a tendency for the third upper and lower 
incisor, the canines, and the first premolars to be reduced 
and disappear, and all intermediate grades are found. 
In the molars there is a regular tendency toward simplifi- 
cation; so that in the upper molars of the earlier forms there 
is a deep inner fold and a more moderate outer fold, either 
or both of which may disappear completely, though in 
one series the fold seems to have been accentuated instead 
of lost. The feet may be adapted to running or hopping. 
In the Deseado and Santa Cruz material, four series 
of modifications may be distinguished which I have desig- 
nated as families; (1) the Avrchaeohyracidae, primitive 
forms in which the incisors are little enlarged, with inner 
and outer folds on the molars, those on the inner side of 
the upper molars being very deep, bulla small, feet un- 
known; (2) Interatheriidae, first upper and lower incisors 
rooted and of moderate size, inflexions on both the inner 
and outer sides of the molars, bulla large, feet adapted to 
running; (3) Hegetothertidae, incisor 1 of upper and lower 
dentition greatly enlarged and rootless, molars simplified, 
bulla large, feet adapted to running or to hopping; (4) 
Eutrachytheridae, large forms with the first upper and lower 
incisor enlarged and rootless, the upper molars with the 
inner fold developed and bifurcated, bulla large, feet 
unknown. 
For comparison of the various genera, they are charted 
on page 55, the dental character being used, as but few 
have the skeleton known, which is especially so of the earlier 
genera. 
From the foregoing chart and the comparative figures of 
the upper and lower dentitions, the variety and at the 
same time the homogeneity of the 7'ypotheria is evident. 
The gnawing front teeth resemble those of rodents, espe- 
