1913] 



Merriam: New Anchitheriine Horses 



423 



prominence arising from the ectoloph extends inward near the 

 outer end of the metaloph but fails to meet that ridge. The 

 outer end of the metaloph tends to swing a little in front of the 

 inner transverse prominence of the ectoloph. 



The inner transverse prominences arising from the ectoloph 

 attain their greatest elongation or height near the summit of the 

 ectoloph, and rapidly diminish in height as they extend toward 

 the base of the tooth. On Dm 4 the transverse prominence con- 

 sists of two small tooth-like projections. On the longer or lower 

 of these points the diameter, parallel with the height of the tooth 

 crown, is not more than twice the anteroposterior diameter. The 

 second projection, situated farther toward the base of the 

 ectoloph, is an exceedingly small tubercle. The smaller projec- 

 tion does not reach the bottom of the valley between metaloph 

 and metacone crescent. On Dm 3 the inner transverse prominence 

 of the ectoloph is very small, and is situated near the crest of 

 the ectoloph. On Dm 2 the prominence is higher, but is reduced 

 rapidly at the proximal end and does not connect with the 

 metaloph. 



A certain significance may attach to the situation of the inner 

 transverse ridge of the ectoloph. In the milk teeth of the 

 Nevada form, this transverse crest or ridge rests upon the pos- 

 terior end of the paracone crescent. In a permanent upper 

 molar (no. 11570) of II ypohippus from the Middle Miocene of 

 Virgin Valley, Nevada, the connection between metaloph and 

 ectoloph is established at the posterior end of the paracone 

 crescent. In the cheek-teeth from P 2 to M 3 in H. osborni the 

 inner transverse ridge of the ectoloph arises almost exactly at 

 the junction of the paracone and metacone crescents. In no. 

 12564, a very narrow H ypohippus tooth from the Virgin Valley 

 Miocene of northern Nevada, the union of metaloph and ectoloph 

 seems to be as in H. osborni. In the original reference to tooth 

 no. 12564 the writer suggested 2 that this form might represent 

 a species distinct from no. 11570, which is much wider antero- 

 posteriorly and shows the more anterior position of the inner 

 transverse ridge of the ectoloph. 



2 Merriam, J. C, Univ. Calif. Publ. Bull. Dept. Geol., vol. 6, pp. 259 and 

 260, 1911. 



