MERYCHYUS 221 



Foramina: The infraorbitals lie above the posterior of P 4 . The supraorbitals are 10 mm. from 

 the mid-line. The posterior palatines are opposite the interval between P 4 and M 1 . The foramen 

 ovale is large. Owing to the size of the bulls and their close approach to the basioccipital, it 

 appears that the foramen lacerum medium has moved forward, almost on a line with, but internal to, 

 the foramen ovale. 



Dentition: Peterson considers P 2 and P 3 as more complicated than the corresponding teeth in 

 M. a. leptorhynchus. The teeth in the latter are in a more advanced stage of wear, and it seems that 

 if both were alike, the differences would not be of importance. The tooth structure in both forms 

 appears to agree remarkably well. The molar-premolar index is 0.74 and in M. a. leptorhynchus 

 0.77. 



Skeleton^ Vertebral formula: C 7, D 14, L 6, S 5-7, Ca 5 (+ ?). Peterson described the 

 skeleton of this species in 1923, and much of the following is taken from his careful work. He said: 



The centra of the anterior dorsals are depressed and broad, as usual, and the arches are heavy, while further 

 back in the series they are higher and narrower and the neural arches lighter. The last three dorsal vertebra are 

 provided with lumbar-like fost-zygafofhyses as in Phenacoccelus, but the last dorsal does not have the transverse 

 process developed to the same degree as in the latter genus. There are no distinct intervertebral foramina of the 

 dorsals as in Promcrycochcerus. 



The inferior ridge of the anterior lumbars is more pronounced than in Merycoidodon, resem- 

 bling more closely those of Phenacoccelus and Promerycochosrus. The ilia are supported by the 

 pleurapophyses of the first of the sacrals. The sacrum is longer than in Merycoidodon. 



Peterson considers the tail to have been quite short, "as is suggested by the lack of the enlarged 

 and convex anterior surfaces of the centra of the proximal caudals usually found in long-tailed 

 animals." 



The anterior ribs are flattened and robust. 



To quote further from Peterson: 



The general outlines of the scapula are more nearly like those of Leptauchenia or Phenacoccelus. The bone 

 as a whole is shorter than in the latter genus, but the acromion process is less developed, and the coracoid border 

 of the blade is more suddenly expanded. There is no metacromion process, as in Merycoidodon or Promery- 

 cochosrus, and in this respect the present genus again suggests Phenacoccelus or Leptauchenia. 



The humerus is quite similar to that of Phenacoccelus in its general proportions. The supinator ridge and 

 the internal epicondyle are smaller, otherwise the differences are only of very minor importance. 



The radius and ulna are very different from those of Merycoidodon and Phenacoccelus ; in fact they differ 

 from most of the Oreodonts by the reduction in the thickness of the ulnar shaft and the broadening of the shaft of 

 the radius. These bones are proportionally longer than in Phenacoccelus, Merycoidodon, Promerycochcerus, and 

 many other forms. The proximal and distal articulations are, however, typically oreodont in their general 

 character. 



_ The forefoot is high and narrow, when compared with most other oreodonts. The lunar, magnum, and in 

 particular the trapezoid are strongly reduced in transverse diameter, while vertically the lunar is considerably 

 increased; in fact the entire carpus appears to be more specialized in the direction of other cursorial forms. The 

 lateral digits are, however, very little, if at all, reduced, when compared with those of Merycoidodon. The 

 phalanges are of the typical broad and flat oreodont type. 



The pelvis is proportionally shorter than in Merycoidodon and the point of the ilium is possibly less developed 

 than in this genus and in Phenacoccelus ; but its transverse diameter is fully equal to that in these genera. The 

 pelvic cavity appears to be broad, but not deep. The acetabulum is deep, and the heavy anterior border has a 

 slight backward curve, in order to more completely lock the head of the femur. There is a well developed ischial 

 tuberosity. The obturator foramen is oblong and of large size, while the pubic symphysis is quite solidly coossified. 

 The two ossa pubis form a broad bony surface, which terminates posteriorly in a v-shaped emargination. 



The femur is relatively slightly longer and slenderer than in Merycoidodon; the lesser trochanter is more 

 directly posterior on the shaft; the fibular border of the shaft is sharper, and terminates distally in a more promi- 



