NOTUNGULATA
Order I. Upper molars composed of an external longitudinal crest and two transverse crests, the posterior the less developed; lower molars composed of two joining crescents with a “pillar” in the posterior crescent; structure of the feet and limbs varying.
Suborder 1. Litopterna: teeth brachydont to hypsodont; lower molars with the anterior and posterior crescents subequal; squamoso-periotic region not inflated; limbs elongated; pes unguligrade; digits 3-3 or 1-1.
Suborder 2. Typotheria: teeth hypsodont lower molars with the anterior crescent shorter than the posterior; squamoso-periotic region inflated; limbs elongated in varying degrees; pes plantigrade to semi-plantigrade; digits 5-4 or 4-4.
Suborder 3. Toxodontia: teeth brachydont to hypsodont; lower molars with the anterior crescent shorter than the posterior; squamoso-periotic region inflated; limbs short; pes semidigitigrade to digitigrade; digits 3-3.
Suborder 4. Homalodontotheria: teeth brachydont; lower molars similar to those of Toxodontia; limbs moderately elongate; pes semidigitigrade; digits with large curved claws, 5-5.
Suborder 5. Astrapotheria: teeth brachydont to moderately hypsodont; canines enlarged into tushes; molars similar to those of Toxodontia; limbs greatly elongated; feet unknown.