|H. F. Osborn 1918 (Protohippus simus)|| (Gidley, 1907, p. 925) (1) Allied to Protohippus perditus though somewhat larger and differing in the following characters: (2) protocones of a more progressive stage in their fuller development anteriorly, thus forming a deeper infolding of the enamel between them and the protoconules; (3) fossettes narrower transversely than in P. perditus; (4) pre- orbital or facial region relatively shorter and broader than in P. perditus; (5) incisive border but little curved, giving the muzzle a rather truncate appearance; (6) palate broader and less arched than in P. perditus, especially in front of pre- molars; (7) malar fossa wanting, as in P. perditus; (8) lachrymal fossa broader, much more shallow, and less sharply defined than in P. perditus; (9) comparative great vertical depth of skull may be due in part to distortion. (Osborn, 1918) (10) Protocone strongly constricted and separated at the summit from protoconule; (11) evidence of a small pli caballin, of one crochet enamel fold in prefossette, and of two enamel folds in postfossette.|
|H. F. Osborn 1918|| (Leidy, 1869, p. 328, Osborn, 1918) (1) Superior molar teeth bear a resemblance to those of Proto-
hippus perditus; (2) protocone isolated in the unworn crown (Fig. 4); (3) protocone early connected by wear with proto- conule; (4) prominent, simple enamel folds entering pre- and postfossettes from median portion of metaloph; (5) a pli caballin and prominent enamel fold entering prefossette from crochet region; (6) protocone of elongate-oval section, projecting farther inward than hypocone. (Characters based on neotype, Gidley, 1907, p. 890) (7) Size considerably exceeding that of Pliohippus mirahilis. (8) Deciduous premolars of narrower proportions than in P. mirabilis; (9) greater complexity of the enamel foldings in both the milk and permanent series. (10) Malar fossa without dividing ridge, com- paratively smaller and more shallow than in P. mirahilis; (11) basisphenoid proportionately longer than in Protohippus perditus, not overlapped by vomer. (Matthew, 1913, from type and neotype) (12) Deciduous premolars decidedly more hypsodont than in Pliohippus mirabilis; (13) permanent molars with long crowns; protocone united with protoloph except near summit of crown; (15) protocone united with hypocone when teeth are well worn; (16) protocone oval, rather large, anterior in position on premolars; (17) fossette borders with a few enamel folds, disappearing when tooth is about half worn; (18) pli caballin well developed toward the summit of crown.
|J. H. Quinn 1955 (Hippotigris clarendonensis)||Length of tooth row simi- lar to that of H. sellardsi, transverse diam- eter of teeth less than that of H. sellardsi; ram us considerably deeper; metaconid and entoconid more restricted; antero- external angle of metaflexid deeper; pli caballinid on M. 3; parastylids retained on all teeth.|
|J. H. Quinn 1955 (Hippotigris parastylus)||Larger than H. sellardsi; commissure of lower premolars arises from metaconid, which is directed strongly anteriorly and extremely long; antero-external angle of metaflexid prom- inent; median valley penetrating more deeply between the reenti:ants of flexids han in H. sellardsi; hypoconulid molar elongate.|
|J. H. Quinn 1955 (Hippotigris sellardsi)||Size slightly smaller than H. burchelli; upper teeth slightly curved; styles heavy, valleys deep and concave, weakly ribbed; protocones detached at summit of crown, elongate, triangular; postprotoconal valleys shallow; lower teeth with large elongate metaconid an,d reduced, leaf-shaped metastylid, commissure of premolars connected with both; median valley on molars penetrating not deeply between reentrants of the flexids; parastylid prominent; little or no indication of a pli caballinid|
|R. C. Hulbert 1988||Large Protohippus with toothrow lengths of about 130 to 145 mm and unworn molar crown heights of about 54 mm. Larger and more hypsodont than P.perditus, with a broader muzzle, smaller (on average) DP1, more elongate protocone, and more complex fossette plications. Hypoconal groove present until late wear-stages; forms lake on premolars. Protocones isolated from protoselene in early wear-stages of P3-M2, until moderate wear on M3; not as isolated as those of P. gidleyi. Metastylid large, well separated from metaconid by well developed, persistent lingual flexids especially on premolars.|