Vol. 4] Sinclair. — John Day Rodents and Ungulates. 



135 



Specific characters. — Facial region shortened, profile concave. 

 Forehead convex. Molars rapidly increasing in diameter pos- 

 teriorly, with strongly mammilated enamel. M 3 with large 

 antero-internal style and prominent postero-internal heel. In- 

 ferior dental series interrupted by a diastema on either side of P 2 . 



Two speciemens have been referred to this species. In addi- 

 tion to the imperfect cranium associated with the mandible which 

 has been taken as the type of the inferior dentition, there is in 

 the University collection a much more perfectly preserved cran- 

 ium (No. 5^6) collected by Mr. L. S. Davis from the middle 

 of the Diceratherium beds below Sheep Rock, in Butler Basin, 

 Grant Co., Oregon. This specimen, combined with mandible No. 

 T9K9 is figured on Plate 16. 



Generic position. — Thinohyus decedens was placed by Cope 

 in a distinct genus, Chaenoliyus, which he distinguished from his 

 Bothrolabis by the presence of three superior premolars in the 

 former and four in the latter. The type of Chaenoliyus has been 

 examined by Professor Merriam, to whose notes the writer is in- 

 debted for the following particulars. The first superior pre- 

 molar is a double-rooted tooth situated inside the base of the 

 canine at a distance about equal to its own diameter, and reach- 

 ing about as far forward as the posterior margin of that tooth. 

 Posterior to P 1 , of which the base is preserved, the alveolar bor- 

 der is somewhat broken, but there is a small alveolus close to that 

 for the anterior root of P 3 . The dental formula is therefore the 

 same as in Bothrolabis, and the two genera must be regarded as 

 identical. 



The writer has examined the type of Marsh's Thinohyus, 

 which was proposed* for the reception of two porcine species from 

 the John Day,! 1 , lentus and T. socalis, and is unable to distin- 

 guish Bothrolabis from it. Bothrolabis is therefore bracketed as 

 a probable synonym. An imperfectly preserved mandible (No. 

 1990) from the the Diceratherium beds near Price, in the 

 Crooked River Basin, Crook Co., Oregon, corresponds fairly well 

 in the spacing of the lower premolars and in size with T. lentus 

 and has been referred to that species in our list of the fauna 

 from the Diceratherium beds. 



* O. C. Marsh, Am. J. Sci., 3d Ser., Vol. IX, pp. 248-249, 1875. 



