In the Deseado and Santa Cruz material, four series of modifications may be distinguished which I have designated as families; (1) 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 unknown; (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. CHART OF TYPOTHERIA ============================================================================ | Age |Formula|Canines | U. Molars |Last i.| Toes | | | | | | molar | | --------------+-------+-------+--------+------------+-------+--------+------ Hegetotherium | Santa |3 1 4 3|vestigal|no inner |3-lobed|cleft | | Cruz |-------| |fold | | | | |3 1 4 3| | | | | --------------+-------+-------+--------+------------+-------+--------+------ Prohegeto- |Deseado| | |no inner | | | therium | | | |fold | | | --------------+-------+-------+--------+------------+-------+--------+------ Pachyrukhos |Santa |1 0 3 3|lacking |no inner |3-lobed|not | | Cruz |-------| |fold | |cleft | | |2 0 3 3| | | | | --------------+-------+-------+--------+------------+-------+--------+------ Propachyrucos |Deseado| |slightly| |3-lobed| | | |-------|reduced | | | | | |3 1 4 3| | | | | --------------+-------+-------+--------+------------+-------+--------+------ Prosotherium |Deseado|1 0 4 3|lacking |simple |3-lobed|slightly| pm. | |-------| |inner | |cleft |simple | |2 0 4 3| |fold | | | --------------+-------+-------+--------+------------+-------+--------+------ Archaeophylus |Deseado|? 1 4 3|rather |deep inner | | |