196 the merycoidodontim: 



M. culbertsonii. The caudal vertebra; are very different from those in Merycoidodon and Eporeo- 

 don, resembling most closely those of Sus, that is, they are very unlike any ruminant tail. Matthew 

 (1901 A, p. 415) describes them thus: 



The centra are small and rather short, the anterior zygapophyses quickly becoming rudimentary, the posterior 

 ones uniting to form a small process projecting over the next succeeding vertebra, while the transverse processes 

 dwindle to small pointed median lateral projections, and the proximal and distal lateral processes do not appear. 



The ribs are stout, and apparently the animal had a good-sized rib basket. 



The humerus is wider and stouter than that of Eporeodon, although of about the same length. 

 The radius and ulna are separate and very markedly curved, and the ulna has a short but very heavy 

 olecranon. The manus is short and wide. Matthew (p. 417) compares the hind leg and foot of 

 T. rusticus with those of Merycochcerus proprius as follows: 



The hind limb differs from that of M. proprius in the shorter leg and longer foot, with metatarsals less 

 closely bound together and the lateral ones more reduced, also probably in the longer phalanges and narrower 

 unguals. 



The femur is one sixth shorter in proportion than that of M. p-oprius, with heavier, more angulate shaft, 

 the low ridge from the external epicondyle extending up the shaft more than two-thirds the length of the bone. 

 Lesser trochanter less prominent but more massive and rugose. 



The tibia is one-tenth shorter in proportion, shaft less rounded, cnemial crest equally high but not extend- 

 ing so far down. Distal end much more flattened, rectangular in cross-section, twice as wide as deep, concave in 

 front, a broad triangular surface for the fibula on external side and a broad, blunt malleolus on the internal, the 

 trochlea shallow and wide and very limited anteroposteriorly. 



The metatarsals are one-fifth longer in proportion (their absolute length greater than in M. proprius), much 

 slenderer in the shafts, the lateral pair more reduced; the facet for the second cuneiform on mt II is very small, 

 and that for the internal cuneiform nearly proximal, instead of lateral as in M. proprius. 



Discussion: That this species of Leidy's does not belong in Merycochcerus seems certain now, 

 but Leidy (1870B, p. 109) was puzzled about where it did belong, for he thought that possibly it 

 was the same as the type of Merychyus medius, or, again, that T. rusticus was the female of 

 Merycochcerus proprius. Since he did not feel sure of either idea, he was disposed to place these speci- 

 mens in a new species. In 1873 (p. 205) Leidy thought probably that the T. rusticus might belong 

 to the same genus as Metoreodon {Merychyus) major, since the corresponding third superior 

 premolars are very much alike. 



Scott (1890B, p. 348), referring to this species as Merychyus (? Merycochcerus) rusticus, wrote 

 that the enclosed groove from the two premaxillaries is completely obliterated, and the condition of 

 the anterior narial opening calls to mind the condition in Tapirus. 



Matthew (1901 A, p. 418) concluded that this species differed from its nearest relative, 

 Merycochcerus proprius, in the longer and lower skull, more bulbous frontal region, shorter limbs, 

 and longer, more spreading, but less tetradactyl, feet. Furthermore, he believed that it stood inter- 

 mediate between M. proprius and the other merycoidodonts, approximating Merychyus in some 

 characters and Promerycochcerus in others. On page 397 of the same reference he used the sub- 

 generic name Brachycrus, without definition, for the reception of rusticus. 



Douglass (1907B, pp. 87, 96) believed that T. rusticus might belong to a different genus than 

 Merycochcerus, and he compared Pronomotherium laticeps with T. rusticus, stating that these two 

 forms resemble each other much more closely than do the former and M. proprius. Of the first two 

 species he wrote more in detail as follows: 



The symphysis of the premaxillaries, the concavities of the sides of the face, the way the infraorbital foramen 

 opens, the sudden widening of the skull at the anterior portion of the zygomatic arches, the reduction in the size 

 of the incisors, and the form of the chin and other portions of the mandible are much the same in both, yet there 

 are slight differences in all of these. 



