140 



University of California Publications. 



[Geology 



median ridge on their external faces. The lower premolars are 

 unspaced and are separated by a long interval from the canine. 

 All the anterior premolars are double-rooted. Unlike the su- 

 perior series, the molars increase in length posteriorly There 

 was at least one prominent cusp on the heel of M... External, an- 

 terior and posterior cingula are developed on the molars. 



Affinities. — Thinohyus osmonti may be readily distinguished 

 from T. subaeqxians by the absence of spacing between the lower 

 premolars and the presence of a long diastema in front of T 1 . In 

 T. subaequans the inferior diastemata are short and are anterior 

 and posterior to P,. The length of 1VP-M 3 is also considerably 

 greater in that species and the molars increase somewhat in di- 

 ameter posteriorly. T. pristinus is characterized by a large heel 

 on M 3 separated from the posterior cones by a valley. In the 

 mandible of this species, P 1 is separated from P, and the canine 

 by short diastemata. In T. triehaenus the first lower premolar 

 is close to the canine and there are short intervals immediately 

 anterior and posterior to P 2 . In both T. pristinus and T. tri- 

 ehaenus the superior dental series is considerably longer than in 

 T. osmonti. T. rostral us differs not only in the greater length of 

 the superior dental series, but in having a longer M 3 , on which the 

 heel is extended particularly along the inner side, being nearly 

 in a straight line with the inner border of the protocone and 

 hypocone. It differs also in the spacing of P 2 and P 3 which are in 

 close series in T. osmonti. These species are all of the type with 

 elongated facial region. The short-faced T. decedens differs 

 from T. osmonti in the concavity of the facial profile, the con- 

 vexity of the forehead, the increase in size of the superior molars 

 posteriorly, the rugosity of the enamel on the molars, the charac- 

 ter of M 3 , and the spacing of the lower anterior premolars. In 

 the mandible of the more imperfectly known T. lent us, P x is 

 separated from the canine and P 2 by diastemata of respectively 

 8mm. and 10mm. 



Measurements. 



'Length of skull from premaxillae to foramen mag- No.SlKJ 



mini 225 mm 



Length of mandible from anterior extremity to con- 

 dyle inclusive 186 



Length P'-jNF inclusive ■ 79.5 



