3S4 Bulletin American Museum of Natural History. [Vol. XXVII, 



and calcaneum approximating the Meniscotheriuin type (in 

 TrigonostyJops). e. g., Trigonostylops, Notostylops. 



c. Size large; molars subtriangular. Astragalus with trochlea 

 flattened but wide, head small, convex, neck oblique. Ungues 

 compressed, fissured. Homalodotherium. 



?fi. Molars more quadrangular, partly suggesting those of Astra- 

 pother iimi. Leontinia. 



?e. Molars approximating the Leontinia type; astragalus and 

 calcaneum approximating the Nesodont type, Colpodon. 



2. Suborder Astrapotheria. x\stragalus probably flattened, i. e., 

 losing head and neck. Fibular facet on calcaneum probably re- 

 duced (Gaudry). Limbs rectigrade (Gaudry). Squamoso- 

 periotic region not inflated; zygomatic arch not extending to top 

 of occiput. ? Canines enlarged. 



?1. Molars moderately hypsodont, suggesting partly the Homalo- 

 dothere type: e. g., Alhertogaudrya. 



2. Molars approximating the Astrapotherium type: e. g., Astrapo- 

 notus. 



3. ?Canines tusk-like, molars very hypsodont, premolars small. 

 Astrapotheriuvi. 



3. Suborder Toxodontia. Astragalus with distinct neck and convex 

 head, trochlea more or less compressed. Fibular facet on cal- 

 caneum prominent. Squamoso periotic region inflated. Zygo- 

 matic arch extending backward to top of occiput. Median incisors 

 often enlarged as cropping teeth, canines often reduced, never 

 tusk-like. Premolars well developed. Upper molars curving 

 more or less inward above and outward below; with strong crista 

 and large protostyle and hypocone, flanking the centrally placed 

 protocone. 



Interatheriidse, Hegetotheriidse. 



Archseohyracidfe. 



Typotherium. 



Nesodontidfe, Toxodontidse. 

 III. Order Litopterna (?derived from I). Upper molars more or less 

 hippoid, /. e. buno-lopho-selenodont, with mesostyle; brachyodont to 

 hypsodont. Lower molars with subequal trigonid and talonid. Feet 

 unguligrade. Carpus a modification of the serial type (lunar separated 

 from unciform by cuneiform-magnum contact). Tarsus with distinct 

 fibulo-calcaneal facet; astragalus becoming hippoid, i. e., with broad 

 transversely convex head (navicular facet). Digits long, tri- to mono- 

 dactyl. Squamoso-periotic region not inflated; zygomatic arch not 

 extending on top of occiput. Fam. Proterotheriidse, Macraucheniidse. 



