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; (i) the Archaeohyracidae, 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) Hegetotheriidae, 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 Typotheria is evident. 

 The gnawing front teeth resemble those of rodents, espe- 



