THE PALEONTOLOGIC RECORD 77 



distinct, at first sporadically along with the more primitive ones, then 

 more abundantly, finally replacing the older ones altogether. The 

 intermediate gradations occur along with the more typical individuals, 

 but without much definite relationship to intergradation in the succes- 

 sion of strata. 1 



We may illustrate from the evolution of the oreodonts, as these are 

 the most abundant and most completely known of American fossil 

 mammals. 



The earliest known representatives of the phylum are Protoreodon 

 and Protagrioclicerus from the Upper Eocene Uinta beds of Utah. Both 

 have very short crowned teeth with five crescents on the upper molar, the 

 fifth crescent quite distinct. The fourth premolar is not molariform. 

 For the next stage we have to shift to another formation, 400 miles 

 away, the White Eiver. In the lowest strata of this formation, the 

 Titanotherium beds, we find Oreodon, Bathygenys and Agriochcerus, 

 all with decidedly longer crowned teeth, and no trace of the fifth cres- 

 cent in the molars. In Oreodon and Bathygenys the fourth premolar 

 is non-molariform, composed of one inner and one outer crescent, as 

 usual among Artiodactyls. In Agriochcems it has become imperfectly 

 molariform with two outer crescents and one inner one. Between the 

 Uinta and White River oreodonts a sharp break intervenes and no 

 intermediates are known. From this point we can trace the subphyla 

 of oreodonts up through a considerable succession in the Big Badlands 

 of South Dakota and the adjoining region. Oreodon culbertsoni, 0. 

 bullatus, Eucrotaphus, Eporeodon, Mesoreodon and Merychyus appear 

 to be approximately successive stages in specialization. The skull is 

 shortened, the teeth become longer crowned, the tympanic bullse are 

 enlarged, lachrymal vacuities appear, the limbs are lengthened, the 

 feet lengthened and compacted and the thumb is lost. But there is 

 not a continuous intergradation in any of these features as we pass 

 upward in the beds. Oreodons with small bullse are abundant in the 

 lower and middle White Eiver, the bullae varying very little in size. 

 A species with medium-sized bullse occurs occasionally associated with 

 them. In the Upper White River all the oreodons that I have seen have 

 bullae of large size. The size of the bulla, then, does not increase con- 

 tinuously as we go up through the formation. Another and much 

 more specialized genus of oreodonts, Leptauclienia, suddenly appears 

 in abundance in the Upper White River. I have seen a single specimen 

 of this genus from the Middle beds, but it shows no more primitive 

 features than those of the Upper beds. In the Lower Rosebud, immedi- 

 ately overlying the White River, species of Eporeodon are common, like 

 1 The statements of fact herein contained are based partly upon field experi- 

 ence, chiefly upon the records of some 20,000 specimens of fossil mammals and 

 reptiles in the American Museum collections, most of which the writer has had 

 occasion to examine and identify and to post the field records of level and 

 locality, in the course of cataloguing work. 



