76 SINCLAIR— HYRACODONS FROM THE 



This is probable, as shown by our No. 12679, but I do not feel justi- 

 fied in regarding the well-worn premolars of our No. 10144 as per- 

 sistent milk teeth and so have accepted without question Mr. Troxell's 

 species. ^^ Some doubt seems to exist about the stratigraphic position 

 of his type specimen which is stated to be from the Middle or Lower 

 Oligocene, Crow Buttes, South Dakota. The first-mentioned Prince- 

 ton specimen occupies the stratigraphic position which might be ex- 

 pected for such an advanced form, which, so far as anyone can say 

 at present, may range into lower beds and even occur in the Lower 

 Oligocene, a question which can be settled only by further collecting. 

 The second Princeton specimen was obtained by the Expedition of 

 1882 and is from the White River Oligocene of South Dakota, occur- 

 ring in a matrix resembling that of the Leptauchenia beds, but no 

 further information is available regarding its stratigraphic position. 

 As there are but three specimens of Hyracodon leidyanus so far 

 available, which agree closely in dimensions, no series of size variants 

 can be recognized. 



Fig. 7. Hyracodon sp. No. 3758 United States National Museum. First 

 to third milk premolars of the right side, crown view, three fourths the natu- 

 ral size. No germs of the replacement series have formed above these teeth 



Of the other species discussed by Mr. Troxell, "Hyracodon" 

 planiceps^^ appears among the rhinocerotidse in Dr. Matthew's faunal 



1^ The milk dentition shown in Fig. 6 has the crown-patterns of the pre- 

 molars more obliterated by wear than in m^ and the necks of these teeth are 

 farther below {i.e., ventral to) the alveolar border than is true of the first 

 molar, showing that they were erupted and in use before the appearance of 

 that tooth. In Nos. 10802 and 10144 (Figs. 4 and 5), the first molar is 

 abraded to a greater degree than the two preceding premolars and the necks 

 of the tooth crowns are approximately on the same level, demonstrating that 

 the premolars appeared after the eruption of m ^ and are of the replacement 

 series. Mr. Troxell's drawmg of H. leidyanus shows m ^ more extensively 

 abraded than the premolars, so that their character as permanent teeth is fully 

 established. Dr. Matthew has kindly called my attention to these criteria. 



18 W. D. Matthew, " Cenozoic Mammal Horizons of Western North 

 America," Bulletin 361, U. S. G. S., in collaboration with Professor Osborn, 

 p. 105, 1909. 



