PROTOHIPPUS. 



127 



GENUS PROTOHIPPUS Leidy 1858. 



Proc. Acad. Nat. Sci. Phila. 1858, pp. 2(5-27. 



Genotype. — Protohippus (Equus) perditus Leidy, from the Niobrara River, near Fort Niobrara, Procamelus-Hipparion 

 zone, Lower Pliocene, Nebraska. 



The twelve species which have been referred to the generic stage of Protohippus, beginning with the P. perditus Leidy 

 of 1858 and ending with the P. secundus O.sborn of 191S, are known within a relatively short geologic period, namely, the 

 Procamelus-Hipparion zone, now regarded as the typical American Lower Pliocene, along the Niobrara River, Nebraska, 

 along the Little White River, South Dakota, in the Clarendon formation of Texas, in the Tehuichila of Vera Cruz, Mexico; 

 also from the somewhat more recent and doubtfully separated Peraceras zone of the Republican River, Driftwood Creek, 

 Nebraska. 



This is in wide contrast with the prolonged geologic extent attributed to Mesohippus, Miohippus, Parahippus, and 

 Merychippus, as well as to Pliohippus which is recorded from its first appearance in the Lower Pliocene, Procamelus- 

 Hipparion zone, to the full Middle Pliocene, Glyptotherium zone of Mt. Blanco. 



The principal distinction between Merychippus and Protohippus is that in the latter the milk molars are narrow, 

 subhypsodont, fully cemented, inner basal cusp small or lacking, the permanent teeth with higher crowns. 



The characters which are common to the species listed above are: 



1. Grinding teeth hypsodont, fully cemented at the time of eruption. 



2. Deciduous premolars subhypsodont, fully cemented at time of eruption. 



3. Proto- and metaconules crescentic, united with crochet and hypostyle respectively. 



4. Protocone round or oval, always united with protoloph. 



5. Protocone and hypocone typically symmetrical, sometimes united after wear, protocone sometimes slightly 

 enlarged. 



6. Enamel borders of fossettes relatively simple or with primary plications only, e. g. pli caballin, pli crochet, pli 

 hypostyle, pli metaloph, etc. 



7. Metastylid well separated except at base of crown, where it fuses with metaconid. 



S. Lachrymal fossa usually shallow (P. perditus, P. si mux, P. niobrarensis) ; no malar fossa. 

 9. Grinding teeth sharply incurved. 

 10. Feet anisotridactyl, lateral digits abbreviated, metapodials slender. 

 The proposed grouping of Protohippus by Matthew (1913) is as follows: 



Group I 



Protohippus perditus and closely allied P. 

 castilli with P. parvulus fairly distinct; 

 P. pachijops a larger and more progres- 

 sive species. 

 Crowns moderately long, grinders curved, 

 protocone well united to protoconule, 

 fossette borders not much complicated, 

 fossettes somewhat contracted. 

 Protohippus perditus 

 profectus 

 " retrusus 



Group II 



Protohippus placidus, with comparatively 

 long-crowned teeth, upper molars near- 

 ly straight, protocone oval or lenticu- 

 late, slightly though continuously 

 connected with protoloph, fossettes 

 more contracted, grinding teeth nar- 

 row. Near the H. gratum group of 

 Hip par ion. 



Protohip p us pro placid us 



Group III 



Protohippus simus, larger and more spe- 

 cialized derivatives of the typical P. 

 perditus group (/-*. simus and Plio- 

 hippus fossulatus). 



