94 Bulletin American Museum of Natural History. [Vol. X, 



additional materials of known types, as well as of new allied 

 forms ; (2) a careful consideration of the various steps in their 

 evolution ; and (3) a study of certain osteological characters of 

 the higher selenodont Artiodactyla in its direct bearing upon the 

 transmission of acquired characters. 



The oldest members of Tylopoda which we can determine with 

 certainty are found in the Upper Eocene deposits of the Uinta 

 Basin, which strata, it may be noted, contain the first remains of 

 true selenodont Artiodactyla in this country, a fact originally 

 pointed out by Marsh, who was the first to explore this region for 

 fossil remains. In the preceding deposits of the Bridger Basin 

 the remains of Artiodactyla are found, but are very rare, no truly 

 selenodont types being known. It is true that in several genera, 

 such as Homacodon and Helohyus of Marsh, we have a distinct 

 foreshadowing of the selenodont molar, yet it is not until the 

 Uinta is reached that the true Selenodonts appear. 



Mr. O. A. Peterson, to whom we are largely indebted for the 

 beautiful collection now in the Museum from this horizon, divides 

 these beds into three stages,' which he designates as the lower, 

 middle, and upper Uinta Beds, or, to use his own field designation. 

 Horizons A, B, and C. A large part, if not the entire lot, of these 

 remains were obtained from the lower part of Horizon C, or the 

 upper part of Horizon B, so that their stratigraphical position 

 would be correctly stated to be at least 300 to 400 feet below the 

 top, and at least 800 feet from the bottom of the Uinta formation. 

 No fossils are known from the uj^per levels of Horizon C, nor do 

 we know any Artiodactyla from the Brown Sandstones, 800 feet 

 in thickness, constituting Horizon A. If, therefore, close con- 

 nections between these Uinta forms and the preceding Bridger 

 species, on the one hand, and the succeeding White River, Oiigo- 

 cene species, upon the other, are not shown to exist, the fact is, 

 in all probability, due to our lack of knowledge of the species 

 which lived during the time of deposit of these intermediate 

 strata. 



The identification of these Uinta Cameloids is attended with 

 more or less difficulty, owing in part to the fragmentary condition 



' See Osborn's ' Fossil Mammalia of the Uinta Basin,' Bull. Am. Mus. Nat Hist Vol 

 VII, p. 74. 



