192 THE MERYCOIDODONTID^ 



Specific Characters: What there is of this specimen comes nearest to T. zygomaticus in 

 size, but the ramus is not so deep below the tooth row. This may be the female phase of the 

 genoholotype. 



Dentition: The tooth patterns are typical of the genus. In this species the premolar series are 

 somewhat more crowded and shortened than in the earlier forms, and there is a more marked hypso- 

 donty, both of which characters are apparently correlated with an advanced stage in this geologically 



A.M.I4057 



Fie. 140. — Ticholeftus hypsodus Loomis. Mandible and dentition. HT. Cat. No. 14057 A.M.N.H. 1/2 nat. size. 



(After Loomis, 1924.) 



younger form. P 3 and P 4 have the bifurcated anterior crest, and the posterior basin of P 4 is not 

 completely enclosed until the tooth reaches an advanced stage of wear. Externally Mi and M 2 

 have a cingulum in the gap between the two lobes. The inferior molar-premolar index is 0.77. 



Ticholeptus obliquidens (Cope) 1886 

 PI. XXIX, figs. 5-6 



Original Reference: On two new species of three-toed horses from the upper Miocene. Proc. Amer. 

 Philos. Soc, XXIII, pp. 359-360 (Merycochaerus obliquidens). 



Type Locality: Cottonwood Creek, Grant County, Oregon. 



Geological Horizon: Upper Miocene (Mascall). 



Type: Holotype, Cat. No. 8192 A.M.N.H., nearly complete lower jaw, collected by the late Dr. J. L. 

 VVortman. 



Specific Characters: The anterosuperior part of the symphysis is broken away, but the chin 

 profile appears to be nearly straight and has a symphysiodental angle of over 50°. The mental 

 tubercle is prominent. The horizontal ramus is nearly uniform in depth below the tooth row, 

 except for a gradual posterior descent to the rounded angle. The posterior border of the ascending 

 ramus has the nearly vertical outline typical of this genus, and the condyle is moderately inset. The 

 masseteric fossa is small but moderately deep. 



Dentition: The premolars are so crowded that P 2 is obliquely placed. The incisor alveolar 

 parapet is missing. The metastylid of M 3 is large. The molar-premolar index is 0.77. 



Discussion : This species is exceedingly close to T. hypsodus, and both were collected in equiva- 

 lent horizons. I do not believe, however, that they should be considered synonymous but rather 

 that their similarity is due to parallel evolution. Douglass (1901 A, pp. 81-82) tentatively referred 

 a very fragmentary lower jaw, with Mi, to this species. It was collected in the Madison Valley beds. 

 The material is not sufficiently diagnostic for any conclusions as to its proper classification. 



