74 



OSBORN: OLIGOCENE, MIOCENE, PLIOCENE EQUIDvE. 



GENUS PARAHIPPUS Leidy, 1858. 

 Proc. Acad. Nat. Sci. Phila., 1858, p. 2(3. 



Genotype. — Parahippus (Anchitherium) cognatus Leidy, from the Nebraska formation, Procamelus-Hipparion zone, 

 of the Niobrara River, Nebraska. 



Synonyms. — Desmatippus Scott, genotype, Desmatippus crenidens, Amer. Naturalist, Vol. XXVII, July, 1893, p. 

 661, from the Deep River formation, Deep River valley, Middle Miocene, Montana. Altippus Douglas, genotype, Para- 

 hippus {Altippus) taxus, from the Upper Oligocene or Lower Miocene of Woodin, Divide Creek, Silver Bow County, 

 Montana. 



Eighteen species now referred to the genus Parahippus, described between 1858 and 1918, are included within this 

 stage of evolution, which is closely connected with that of Miohippus below and of Merychippus above. The geo- 

 logic distribution is an extremely long one, extending from the diminutive Parahippus pristinus and the P. tyleri of the 

 Promcrycochcerus zone of the Lower Rosebud and Lower Harrison of South Dakota and western Nebraska, through the 

 Merycochcerus zone of the Upper Harrison, in which Parahippus is very abundant, through the entire Tieholeptus-Mery- 

 chippus zone of the Middle Miocene, and up into the Procamelus-Hipparion zone of the Lower Pliocene where the geno- 

 type Parahippus cognatus is said to occur, recorded with remains of such progressive horses as Protohippus, Pliohippus, 

 and Hipparion. The explanation of this survival is that Parahippus, like Hypohippus, had a local adaptive radiation 

 and distribution of its own, favoring a browsing country adapted to its relatively short-crowned grinding teeth. 



1. Grinding teeth structurally intermediate between Miohippus and Merychippus stages. 



2. P 2 " 4 invariably of greater length than m 1 " 3 . 



3. P 3-4 invariably broader than m 1 . 



4. Deciduous and permanent grinding teeth braehyodont to subhypsodont. 



5. Cement thin or wanting, late in development, progressively heavy (P. avus). Enamel surface rugose. 



6. Protocone and hypocone relatively large as compared with the simple lophoid protoeonule and metaconule. 



7. Metaloph with crochet progressively increasing in size (P. cognatus, P texanus). 



8. Anterior enamel wall of metaloph ptychoid or crenulate in certain species (P. crenidens, P. cognatus). 



9. Pre- and postfossettes (cement lakes) tending to close through development of crochet and of hypostyle. 



10. Hypostyle prominent, subtriangular. 



11. Metaconid-metastylid column and hypoconid-hypostylid column of inferior grinders broadening. 



12. Lachrymal fossa vertically broad, shallow, and separate; malar fossa not present. 



13. Facial region always greatly exceeding cranial region. 



14. Orbit closed posteriorly. 



15. Digits of manus and pes anisotridactyl, i. e. lateral digits considerably reduced. 

 Matthew (1913) has suggested the division of Parahippus into three groups, as follows: 



Group I 



Parahippus cognatus Upper Miocene 

 " aims Middle " 



nebrascensis Lower " 

 " texanus " " 



tyleri 



Distinguished by relatively short-crowned 

 teeth, shallow fossa 1 , heavily proportioned 

 limbs, feet moderately elongate, ungual 

 phalanges short and small. 



Group II 



Parahippus pawniensis Middle Miocene 

 " crenidens " " 



" pristinus Lower " 

 " pawniensis atavus " 



Teeth short-crowned, simple, crochet 

 simple, incipient accessory foldings of 

 metaloph, size small, fossae rather deep, 

 limbs slender, moderately long, phalanges 

 short, ungual phalanges long and narrow. 



Group III 



Parahippus coloradensis Middle Miocene 

 " " prweurrens, Lower 



Miocene 



Teeth long-crowned, crochet well de- 

 veloped, with accessory minor crests, ex- 

 ternal ribs prominent, feet long, slender, 

 lateral digits greatly reduced; phalanges 

 of moderate length. 



Thus according to Matthew's observations of 1913 Parahippus divides into at least three distinct parallel phyla dis- 

 tinguished by the characters of the grinding teeth and of the preorbital fossae, and by the stout or relatively slender limbs. 



