MUSEUM OF COMPARATIVE ZOOLOGY. 75 



and, as compared with the metacarpus, their greater length, for the three pha- 

 langes of the fourth digit are together as long as the metacarpal. The unguals 

 are of the same general shape in the two genera, but broader, more depressed, 

 and with the ends less pointed in Merycochcerus. 



The pes in the species of Merycochcerus from the John Day and Deep River 

 beds differs less from that of Oreodon than does the manus, but the species be- 

 fore us appears to show an important departure from that type. The astragalus 

 is shorter, broader, and more massive than in Oreodon, and the distal trochlea 

 has a broader surface for the cuboid. The calcaneum is not preserved. The 

 cuboid is low and broad; the surface for the astragalus is broader than that for 

 the calcaneum, reversing the proportions seen in Oreodon; the calcaneal surface 

 is also of a different shape, as it does not project outwards, and its external mar- 

 gin is straight, not rounded ; unlike the pigs, this facet is not notched on its 

 outer margin. The astragalar surface is not so deeply concave as in the White 

 River genera, and another difference lies in the presence of a broad shallow 

 groove which separates the articular surface into anterior and posterior portions. 

 The peroneal sulcus is shallow. The distal end is almost entirely taken up by 

 the large facet for mt. IV., that for mt. V. being very small and more lateral 

 than distal ; in Oreodon it is entirely distal. The navicular does not differ 

 sufficiently from that of the earlier genera to require description. The ento- 

 cuneiform is relatively large, and in general resembles that of Oreodon, but has 

 a larger bearing upon mt. II. The ento- and meso-cuneiforms are missing, but 

 they were doubtless ankylosed together as in all the other members of the 

 family. 



As a whole, the tarsus has changed in an opposite sense to the changes in 

 the carpus, having become lower and broader, while the carpus has become 

 narrower and remarkably high. 



The metatarsus is suilline in general appearance ; the median digits are short 

 and massive, while the laterals are reduced, especially in length, being not only 

 proportionally but absolutely shorter than in Oreodon Culbertsoni. Mt. II. has 

 an exceedingly small surface for the mesocuneiform, but the head is not oblique 

 as in Sus. Mt. III. has a minute facet upon the tibial side of the head, which 

 appears to encroach upon the mesocuneiform ; and if this is the case, we have 

 here the beginnings of an adaptive reduction of the pes, which is not known to 

 occur in any other member of the family. Except for its heavier propor- 

 tions, mt. IV. is like that of Oreodon; mt. V. has a smaller, more concave and 

 obliquely placed facet for the cuboid than in the latter genus. 



Merycochcerus and Merychyus thus agree with each other, and differ from 

 other oreodonts in which the foot structure is known in the adaptive reduction 

 of the manus, and it is interesting to note that this adaptive method has been 

 independently assumed in several distinct lines of artiodactyles, e. g. the true 

 ruminants, the pigs, and the camels. A study of the oreodonts shows that they 

 are not closely connected with any existing artiodactyles, and it is difficult to 

 see how the same result could be so often reached independently, unless it be 

 the effect of the similar mechanical conditions to which the extremities are 

 subjected. 



