98 



OSBORN: OLIGOCENE, MIOCENE, PLIOCENE EQUIDyE. 



GENUS MERYCHIPPUS Leidy 1857. 

 Proc. Acad. Nat. Sri. Phila. (1856) 1857, p. 311. 



Genotype. — Mcrychippus insignis Leidy, see page 122, from the middle Upper Miocene of Bijou Hills, Brule County, 

 South Dakota. 



The types of the twenty-five species referred to this stage of equine evolution are widely distributed geographically 

 and first appear geologically in the Middle Miocene of the Mascall* Oregon, and of the Deep River, Montana, in the Sheep 

 Creek, western Nebraska, the lower Monolith Series, Tehachapi, California, the Virgin Valley, Nevada, and the Pawnee 

 Creek, northeastern Colorado. There are late stages of Parahippus evolution, some of which (P. brevidens) are transi- 

 tional to Meryckippus. The Mcrychippus stage persists through the upper Middle Miocene into the Lower Pliocene and 

 even into the supposed Lower Pliocene of Snake Creek where progressive stages of Meryckippus (M. calamarius, M. pro- 

 parvulus, M. republicanus, M. patruus) anticipate species of the Protohippus, Pliohippus, and Hipparion stages. 



This generic stage includes many stages of progressive evolution in the genesis and development of single characters, 

 such as the crochet, hypostyle, plications of metaloph, preorbital fossa?, also many advancing stages of proportional evo- 

 lution, such as the elongation of the facial region, elongation of the limbs, reduction of the lateral digits, and especially 

 the elongation, or hypsodontism, of the grinding teeth. This " single character" and "proportional character" evolution 

 is complicated by the presence within the so-called genus Mcrychippus of several distinct phyletic series, some of which 

 lead toward the Protohippus stage, others toward Pliohippus, others toward Hipparion, as, observed by Matthew and 

 the author, making Meryckippus one of the most complex groups of distinct evolutionary phyla at present included 

 within the confines of a single genus. It may well be split up in future into several subgenera when these phyla can 

 be clearly and sharply distinguished from each other throughout the entire geologic period of its occurrence; but such 

 splitting up is inadvisable until a very large number of "species," which are contemporaneous in the respective geologic 

 levels and doubtless largely synonymous, can be either merged into each other or clearly distinguished. 



The prevailing characters of this comprehensive genus are as follows: 



1. Facial region exceeding cranial region, premolar series exceeding molar series. 



2. Grinding teeth subhypsodont (Parahippus brevidens) to strongly hypsodont (Mcrychippus calamarius), upper 

 grinders always strongly curved but molars less than premolars. 



3. Grinding teeth assuming the crown pattern of Equus, (a) by the crescentic enlargement of the pro to- and meta- 

 conules, (b) by the reduction and partial separation of the proto- and hypocones, (c) by the forward extension and branch- 

 ing of the crochet and hypostyle, and (d) by the plication of the enamel walls of the metaloph. 



4. Superior and inferior milk molars brachyodont. 



5. Protocone round or oval, closely united with protoloph or nearly separated to base. 



6. Pre- and postfossettes completely enclosed, enamel fossette borders with simple or double plications. 



7. In inferior grinders metastylid well separated from metaconid except near base of crown, where it fuses. 



8. Antero-external cingulum flat, moderately prominent at base, ending abruptly about half way up the crown. 



9. Lachrymal fossa deep or shallow, malar fossa shallow or absent. 

 10. Feet anisotridactyl, lateral digits abbreviated. 



The grouping and phyletic succession of the species of Meryckippus are not at all clear. The following lines of specific 

 succession were suggested by Matthew in 1913. 



Sheep Creek 

 M. isonesus 'primus. 



Mascall 



. M. seversus . 

 " isonesus . 



Republican River, Rijou Hills, 

 Pawnee Creek Niobrara River, 



.M. sphenodus 1M. insignis 



. " paniensis ? " calamarius 



" campestris Pliohippus mirabilis 



" proparvulus Protohippus parvulus 



" cohipparion Hipparion occidenlale 



" sejunctus Protohippus perditus 



