72 SINCLAIR— HYRACODONS FROM THE 



an individual with well-worn teeth, but with a posterior premolar of 

 the blocked-valley type, with deeply grooved inner tooth wall as 

 defined under B, and of a size practically identical with No. 12687 

 (Fig. 2 B) oi the Princeton collection. I, therefore, submit that 

 Leidy's Hyracodon nehrascensis, in this limited sense, is entirely 

 applicable to that sequence of individuals within the genus which has 

 the transverse valley of p* blocked in the manner indicated, and that 

 no adequate grounds exist, or have existed, for discarding the name. 

 Figures 4-8, Plate XIV., of the "Ancient Fauna" seem to represent 

 unworn examples of the same type. 



As in the preceding species, a series of individual variants can be 

 made out, intergrading by increments of not more than four milli- 

 meters between extremes in the length of the molar series, here taken 

 as the basis for comparison, because present completely in all the 

 specimens studied for dimensions. Material from the Princeton col- 

 lection only is used as follows : 



No. 12662, molar series, length 57 



No. 12687, molar series, length 61 



No. 12688, molar series, • length 61 



No. 12666, molar series, length 65 



No. 12680, molar series, length 67.5 



No. 12563, molar series, length 71 



No. 10723, molar series, length 72 



Of these, the first five are contemporary and from a six-inch zone 

 of rusty nodules about sixty feet (sometimes less) above the top of a 

 similar zone affording No. 12563, which, in turn, lies some forty feet, 

 more or less, above the base of the Oreodon beds and constitutes the 

 lower zone of rusty nodules of our Princeton field nomenclature, 

 while the other may be designated as the upper rusty nodular zone 

 of the Lower Oreodon beds. No. 10723, on the other hand, is from 

 well up in the Protoceras-Leptauchenia beds of the Upper Oligocene 

 and is but a trifle larger than its Lower Oreodon beds predecessor. 

 The specimen numbered 12662 (Fig. 2, C) is as small as Mr. 

 Troxell's H. sclenidens, which I regard as an individual variant of 

 H. arc id ens, as already indicated, for exactly the same reasons which 

 induce me to place this small specimen oi H. nehrascensis as a ter- 

 minal size variant in the nehrascensis series. Each conforms to its 

 own structural type, but intergrades by small -size increments with 

 the largest specimens referable thereto. 



