166 THE MAMMALIA OF THE DEEP RIVER BEDS. 



once more become shortened. (3) That the orbit, after retreating backward so as to 

 be almost entirely behind the line of the molar teeth, had again advanced over those 

 teeth. (4) That the zygomatic arches, after having attained an extraordinary degree 

 of size, massiveness and rugosity, had dwindled to proportions even smaller and 

 lighter than those of Oreodon. (5) That the posterior nares had first been pushed 

 back to a remarkable extent, and had again resumed their original position. (6) That 

 the metapodials, after becoming short and massive to a very unusual extent, had 

 attained a degree of length and slenderness which is equally unusual in this family. 

 We have, it is true, already found reason to believe that, in the horses, progress 

 in the main is accompanied by a certain amount of oscillation in the minor details of 

 structure and that even a certain degree of specialization in a direction away from 

 that taken by the phylum as a whole, may be overcome and suppressed, as, for 

 example, in the case of the elbow joint of Mesohippus. Nevertheless, we know of no 

 facts which would justify us in assuming oscillations of such amount as would be 

 involved in the derivation of Merychyus from Merycochcerus. If we reject Mesoreodon 

 from the ancestry of the former genus, then we must admit parallelism in the struc- 

 ture of its manus, and thus, whichever horn of the dilemma be accepted, the fact 

 that " adaptive " reduction of the manus has occurred twice independently within 

 the limits of the family cannot be avoided, for to regard Mesoreodon as in any way 

 descended from Merycochcerus is a manifest absurdity. The simplest and most prob- 

 able conclusion is therefore that Merychyus and Merycochcerus represent two inde- 

 pendent branches of the oreodont stem, which in some respects have paralleled each 

 other, the former not attaining until the Loup Fork the structures which the latter 

 had already developed in the John Day. 



Recent discoveries have also thrown some light upon the relationships of the 

 Leptauchenia series. Leidy ascribed that genus to the White River formation, while 

 Cope believed that it was confined to the Deep River beds, though it had not been 

 found in the typical (Montana) locality of that horizon. In my former paper I fol- 

 lowed Cope's determination, chiefly on the ground that no member of this series has 

 ever been obtained in the John Day beds. It is now proven, however, that Leidy's 

 determination is the correct one. Dr. Wortman informs me that he has found Leptau- 

 chenia in the upper White River beds, and during the past summer (1893) the Prince- 

 ton party found them in great numbers at the same locality. The morphological 

 difficulty, that all three members of the Leptauchenia series were found only in the 

 same horizon is thus removed, but we are still in the dark with regard to the ancestry 

 of this line, which must be sought for in the lower White River beds. We may, 

 however, confidently remove it from all connection with Merychyus. 



