DAKOTA AND NEBRASKA. 301 



The triturating surfaces (figure 9) of the teeth in the specimen bear a near resem- 

 blance to the corresponding ones of the series (figure 5) just mentioned. The central 

 lakes are of extreme simplicity, and crescentoid. Except in tlie last molar, their con- 

 tiguous arms are much prolonged beyond the line of the distal arms. The lakes of 

 the last molar are more capacious than in the others, and partake of the gaping 

 character seen in the -teeth referred to Pi-otoTiippus. 



In the third and last molars the anterior internal column still preserves its distinct- 

 ness. In the two intermediate teeth it has lost its distinct character, and the bottom 

 of the oblique valley which separated it originally from the postero-median column 

 appears on the triturating surface as an ellipsoidal lake or islet. The distinction of 

 the postero-internal column is entirely obliterated in all the teeth. 



The amount of cementum on the exterior of the teeth appears scanty. It is thin 

 on the outer sides, and that of the central lakes is worn into deep hollows, except in 

 the posterior valley of the last tooth. 



The jaw fragment of the fossil is remarkably different in anatomical character from 

 the corresponding part of the maxillary bone of the Horse. 



The malar ridge terminates above the position of the fourth molar tooth (figure 8), 

 as in Protohippus (figure 1), and therefore does not advance as far as in the jaw frag- 

 ment (figure 10) of the young MerycMppus, in which it terminates above the third 

 temporary molar. A similar difference is observed between the young and adult 

 Horse. 



Above the malar ridge, in the fossil under consideration, the maxillary bone is de- 

 pressed in so wonderful a manner that at first view it appears as if the orbit was ad- 

 vanced far beyond the position it holds in Protohippus, Eqims, or Hipparion. The 

 portion of the depression remaining in the specimen actually forms a horizontal con- 

 cavity, looking like the fore part of the floor of an orbit, and measures from without 

 inwardly (from the broken end of the malar process to the broken edge of the bone 

 where it ascended to articulate with the lachrymal bone) sixteen lines. The depres- 

 sion evidently corresponds with that existing in the jaw fragment (figure 10) of the 

 young MerycMppus described in an account of the genus. A depression apparently of 

 the same nature existed likewise in Hipparion mediterraneum, as before mentioned, 

 and in Anckiiherium Bairdi of White Kiver. Among extinct ruminants, such ant- 

 orbital depressions have been noticed in Oreodon a.nd Bootlierium ; in recent ones they 

 are conspicuous in the Deer. 



The fore part of the maxillary antrum, as existing in the Horse and Protohipjyus, 

 appears to have been entirely replaced by the maxillary depression in MerycMppus. 



The outer edge of the infra-orbital foramen, preserved in the specimen, indicates it 

 to be situated above the interval of the third and fourth molars. In the young jaw 



