0. C. Marsh — Description of Tertiary Artiodactyles. 265 



as complete as that of Homacodon. Figure 12 above shows 

 the second and third upper molars of the present species ; 

 figure 13, three upper molars of an old animal ; and figure 14, 

 the lower teeth, all natural size. The astragalus is well shown 

 in figure 17. The upper molars resemble those of Homacodon, 

 but differ in having but two posterior cones. 



Figure IT. — Astragalus of Helohyus plicodon. Natural size, a, front view; 

 b, side view ; c, back view. 



Two other species which probably should be referred to the 

 present genus are Thinotherium validum and Elotherium 

 lentum, described by the writer from specimens found in the 

 same region as the type of Helohyus. The last lower molar 

 of each is shown in figures 15 and 16. It is quite possible that 

 another allied species was described by the writer under the 

 name Stenacodon rarus, but at present the exact relationship 

 between these forms cannot be determined. All are from the 

 middle Eocene of Wyoming. 



Eocene Selenodont Artiodactyles. 



In the autumn of 1870, the writer explored a new Tertiary 

 lake-basin, which he had discovered south of the Uinta 

 mountains, in Utah. This exploration was carried on with 

 great hardship and much danger from hostile Indians, but 

 proved conclusively that this lake-basin was of late Eocene 

 age, and it was subsequently named by the writer, the Uinta 

 basin. The first results of this exploration were published in 

 an article " On the Geology of the Eastern Uinta Mountains,"* 

 a paper that appears to have been overlooked by several recent 

 writers on the subject. This Eocene lake-basin was quite dis- 

 tinct from the older t Bridger basin, north of the Uinta 

 mountains, which the writer also first explored in 1870. 

 These researches further resulted in securing many new verte- 

 brate fossils from each basin. The two were later distinguished 

 by the writer, by names taken from the largest and most char- 

 acteristic animals found in them. The strata of the Uinta 

 basin were thus named the " Diplacodon beds," and those of 

 the northern lake deposits, the " Dinoceras beds." 



*This Journal, vol. i, p. 191, March, 1871. 



