NO. 6 PALEOCENE FAUNAS OF BISON BASIN — GAZIN 21 



rim of Bison basin, W^ sec. 28, T. 27 N., R. 95 W., Fremont County, 

 Wyo. 



Specific characters. — Size nearly intermediate between Pronotho- 

 dectes mattheivi and Plcsiadapis gidleyi, closer to the latter. Pi 

 pressed close to incisor, posterior lower premolars and molar trigonids 

 of more inflated appearance than in P. matthewi. 



Discussion. — The more typical materials of this species are from 

 the ledge locality about a quarter of a mile west of the saddle locality 

 and include about nine jaws besides the type in the collections of the 

 U. S. National Museum. About four specimens in each of the Na- 

 tional Museum and University of Wyoming collections from the 

 saddle locality would seem to represent a variant somewhat smaller 

 in size, although evidently closer to this species than to Pronotho- 

 dectes matthewi. None of the material of this species was found at 

 the more westerly and possibly higher horizons in the basin. 



Like the material referred to Pronothodectes matthewi, that of this 

 species would appear by definition to be Pronothodectes rather than 

 Plesiadapis, because in all specimens where the dental formula can 

 be determined all the lower premolars were represented, Pi and P2 

 being, of course, single-rooted teeth as in P. mattheivi. P. simpsoni, 

 as noted in the diagnosis, is characterized by much larger teeth, about 

 18 to 31 percent larger in length of lower molar series in materials 

 from the ledge locality, and possibly as low as 12 to about 24 percent 

 larger than P. matthewi in the materials referred to P. simpsoni from 

 the saddle locality. 



Pronothodectes simpsoni differs from P. mattheivi, in addition to 

 its greater size, by exhibiting teeth of a more Plesiadapis-Vike appear- 

 ance. This is noticeable in the more typical materials from the ledge 

 locality in the comparatively inflated appearance of the cusps. Per- 

 haps it is more noticeable in the trigonid, which is distinctly less an- 

 teroposteriorly compressed in M2 and M3. The variant from the sad- 

 dle locality overlaps in size range that represented at the ledge locality 

 and is less obviously different from P. mattheivi in size and appear- 

 ance of the cusps, but would seem to be closer to P. simpsoni. A 

 lower jaw (U. of Wyo, No. 1057, pi. 3, fig. 2) of P. cf. simpsoni with 

 P4 to M3 from the saddle locality exhibits a disproportionately long 

 M3 (buccally incomplete), but other specimens with M3 from the 

 saddle show this tooth to be normally proportioned. Mi and Mo in 

 No. 1057 are scarcely distinguished from these two teeth in the smaller 

 specimens of P. simpsoni from the ledge (i.e., U.S.N.M, No. 20770, 



pl-3>fig-3)- 



