1010.] Typotheres and " Proiijpothcres." 377 



Avhil(' the hypost}'le forms tlu> " ])o.sterior pillar." 'I'he Aca'lodu.s molar has 

 in turn the familiar eleinents which appear in their sini{)lest expression in 

 HijrarotJierium and Ectocion. This fact, with others, indicates that all the 

 elements of the complex hypsodont molars of 'ryi)otheres and Toxodonts 

 may be homologized with those of the Eocene Perissodactyls and Condy- 

 larths. 



(3) In the unworn lower molars of Profypotherium (Sinclair, op. cif., 

 p. 74) the anterior limb of the posterior crescent lies externally to the pos- 

 terior (metastylid) extension of the anterior crescent, as it does in Hyrax. 

 In both cases the lower molar pattern while differing in proportion is funda- 

 mentally identical with the Meniscotherium type. 



(4) The manus of Proty pother mm, as figured by Sinclair {op. cif., 

 p. 71) is already specialized in the loss of the pollex, reduction of digit V 

 and enlargement of digit II. It is, however, functionally tridactyl. The 

 carpus differs from that of Meniscotheriuvi in that the centrale is no longer 

 free but is probably represented in the internal process of the scai)hoid; the 

 lunar has a broad contact with the unciform so that the cuneiform is widely 

 separated from the magnum; the latter is small and polygonal in front view 

 (it is large and quadrate in Ilyrcw and Condylarths). The emphasis of 

 the interlocking features (p. 449) in the carpus may be connected with the 

 hypertrophy of the second digit. The carpus as a whole is not high as it is 

 in early Perissodactyls, but low and broad and in so far agrees with that of 

 Meniscotherium. The ungual phalanges are dorsally compressed but ex- 

 pand somewhat on the plantar surface, much less so however than in Menis- 

 cotherium'. 



(5) The tarsus of Proty potherium (Sinclair, o/;. cit., p. 70) is paraxonic, 

 probably in correlation with cursorial habits; Avhile that of the saltatorial 

 Hegetotherium (op. cit., p. 72) was markedly mesaxonic (Sinclair; compare 

 the mesaxonic pes of the saltatorial Dasyprocta). In both families, however, 

 the astragalus is Aveakly attached to the calcanenm, the neck diverging and 

 the head widely separated by the navicular from contact with the cuboid. 

 The latter feature is also seen in the Litopterns and may be an inheritance 

 from the Condylarth conditions. The trochlea is narrow and very large 

 in proportion to the neck. The fibula articulates with the calcaneum, as it 

 does also in I^itopterns. The navicular has a decided postero-internal 

 prominence, analogous to that in many Rodents and Marsupials. 



Conclusions. The evidence available from Sinclair's contributions 

 permits us to infer with considerable probability some of the leading char- 

 acters of the common ancestors of the Toxodonts, Typotheres, Interatheres 

 and Hegetotheres. These were primitive Placentals with a complete dental 

 formula, a clavicle, third trochanter, entepicondylar foramen and penta- 



