E. L. Troxell — Early Pliocene One- Toed Horse. 337 



seen in Equxis but not in Asinus. The parastyle is broad, 

 nearly 4""", and has two sharp corners ; the mesostyle is sharp 

 but not prominent, while the metastyle is merely the rounded 

 corner of the tooth. 



The first true molar of the lower jaw was preserved in good 

 condition, but it had not attained its full development. It is 

 thin, the width being' but one-third the antero-posterior diam- 

 eter (30 mm ) at the crown ; in this respect and also in the great 

 inner extension of the parastylid, it resembles the three-toed 

 horses. In the new species there is no well defined keel or 

 loop antero-exterior to the protoconid. An inconspicuous 

 groove separates the metaconid from the metastylid ; it is 

 sharp but not deep and fades out after running slightly more 

 than half the length of the crown. This is like Protohippus 

 and results from the narrowness of the metaconid-metastylid 

 column (10 mm ) and its nearness to the protoconid and hypo- 

 conid ; the latter are about equal in size. Near the root the 

 entostylid blends into the entoconid and here the longitudinal 

 diameter of the tooth is reduced to 25 mm . There is no cement 

 on the tooth. 



Skull. — The antero-orhital fossa shows distinctly the lachry- 

 mal and malar parts which are commonly seen in Pliocene 

 horses. Although they form a continuous cavity, the two pits 

 are separated by a faint ridge running from the infra-orbital 

 foramen and the posterior border consists of two distinct, over- 

 hanging shelves. 



The presence of a large depression in this region precludes 

 the possibility of long crowned teeth like those of the modern 

 horse, for both would have to occupy the same space. The 

 exact purpose of this pit is not known, but it is generally 

 thought to have been the seat of a scent gland, which, like the 

 larmier of the deer, was peculiar to animals living in a wooded 

 or hilly country. Presumably, as the horses came to live on 

 the open plains they had less need for such a device to assist 

 in recognizing members of the race, but they had greater need 

 for the long grazing teeth ; so that in later generations the pit 

 gave place to the longer crowns, while the increased range of 

 vision in the open country made it no longer necessary to 

 depend on the sense of smell. Even in the life of the indi- 

 vidual it is possible that the pit became somewhat obliterated 

 as the lengthening of the skull made room for the incoming 

 molars; we find the lachrymal and malar pits seemingly best 

 developed in young animals. According to Owen (Anat. of 

 Verts. Ill, pp. 633) the presence of the " maxillary " pit in the 

 antelope is associated with those animals which go in pairs. 



It has been suggested that the antero-orbital pit may have 

 marked the insertion of a muscle in an animal with a rather 



