Page:Popular Science Monthly Volume 77.djvu/480

474 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 succession of strata.

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 Protagriochærus 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 River. In the lowest strata of this formation, the Titanotherium beds, we find Oreodon, Bathygenys and Agriochærus, all with decidedly longer crowned teeth, and no trace of the fifth crescent 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 Agriochærus 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 culberisoni, O. 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 bullæ 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 bullæ are abundant in the lower and middle White River, the bullæ varying very little in size. A species with medium-sized bullæ occurs occasionally associated with them. In the Upper White River all the oreodons that I have seen have bullæ of large size. The size of the bulla, then, does not increase continuously as we go up through the formation. Another and much more specialized genus of oreodonts, Leptauchenia, 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, immediately overlying the White River, species of Eporeodon are common, like