THE MAMMALIA OF THE DEEP KIVEK BEDS. 165 



slight and subtle changes which are all in the direction of Merychyus. If such spe- 

 cies of the latter genus as M. zygomaticus and M. pariogonus be taken into account, 

 the transition from Mesoreodon is seen not to be very great or abrupt, though as 

 regards dentition and skull structure there still remains a considerable gap between 

 the two genera, which is only one of the many signs that point to a hiatus between 

 the lower and upper beds of the Deep Eiver deposits. In the Merychyus species 

 from the lower Loup Fork (upper Deep River), M. zygomaticus and M. pariogonus, 

 the face has become deeper and the cranium shorter and the wing-like posterior pro- 

 cesses of the parietals are reduced; the nasals are shortened and a fontanelle is 

 formed between the frontal, lachrymal and maxillary. The premaxillaries are de- 

 pressed, flattened, and ankylosed at the symphysis. In foot structure, Mesoreodon 

 has already attained the condition of Merychyus, especially if the more slender and 

 elongate foot of M intermedins be regarded. In the dentition the principal change 

 consists in a modification of the premolars and a rearrangement of the adjacent horns 

 of the internal crescents on the upper molars, for Merychyus pariogonus shows that 

 the hypsodont molars have been acquired within the limits of the genus. We may, 

 therefore, provisionally at least, regard Mesoreodon as ancestral to Merychyus, and 

 the line of descent would then be : Oreodon — Eporeodon — Mesoreodon — Merychyus. 



If this view of the case be correct, then the relationship of Merychyus to Mery- 

 cochcerus must be strictly one of parallelism, by which the articulation of the third 

 metacarpal with the trapezoid and the depressed and ankylosed premaxillaries have 

 been independently attained in the two genera. Merychyus has also run parallel to 

 Leptauchenia in the development of facial vacuities and in the disposition of the cres- 

 cents of the upper molars as well as their hypsodont character. Yet, now that we 

 know the skull structure of these two genera, no one could seriously maintain that they 

 are genetically connected, though Leidy's suggestion of such connection was natural 

 enough from the material at his command. To unite Merychyus and Merycochcexus 

 into a single genus, as Leidy proposed in his later work (ISTo. 24, p. 201), a sugges- 

 tion which Bettany adopted (No. 1, p. 262), would be to construct an unnatural 

 polyphyletic group, unless genera are to be artificial assemblages united only by 

 certain common characters, the morphological value of which is unimportant. It 

 must be remembered that Merycochoerus is a much older form than Merychyus, its 

 peculiarities having all been established in the John Day. To derive the latter genus 

 from the former, it would be necessary to make some highly improbable assumptions. 

 (1) We should have to assume that the face had become depressed upon the basi- 

 cranial axis, only to again straighten out and lie in a line with that axis. (2) That 

 the face, after having elongated more than in any other genus of the family, had 



