PHENACOCCELUS 185 



by P4. In Merycoides the premolars are not crowded, and there is practically no overlapping. 

 The anterior parts of the teeth are better developed than in the last-named genus. The molars 

 are similar to those in Merycoides, except that the metastylid on M 3 is larger and more robust and 

 is nearly in line with the axis of the paraconid and metaconid, while in Merycoides its axis is directed 

 more outward. 



Skeleton: This animal was about the size of a domestic sheep, with a long body and the 

 medium type of limbs, but with a tendency to the short, heavy type. 



The vertebral formula is C 7, D 14, L 6, S 5?, Ca 9?. 



Peterson (1928B) has described the osteology of this species admirably, and for further details 

 I refer the reader to his memoir. The atlas has proportionally smaller expanse of the transverse 

 processes and no vertebrarterial foramen, thus differing from Merycoidodon. Also, as Peterson says, 

 "the vertical diameter of the articulation for the axis is greater, while the transverse is less than that 

 of the older type." The axis differs from that of Merycoidodon in that the odontoid process is 

 broader, more depressed, and more distinctly spout-shaped. Anteriorly the vertebrarterial canal has 

 a tendency to be bridged over, apparently a modification from the earlier type. The third cervical 

 has a neural arch proportionally larger. In general the cervicals do not show much deviation from 

 the Merycoidodon pattern. 



The centra of the dorsals have proportionally greater vertical and lesser transverse diameters 

 than in Merycoidodon. The neural spines from the second to the seventh inclusive become gradu- 

 ally smaller and more backwardly directed. The dorsals from the eighth to the eleventh inclusive 

 differ from the preceding in that the spines are more nearly vertical and have a greater antero- 

 posterior diameter at their summits. The twelfth dorsal has less perfectly interlocking prezyga- 

 pophyses than in Merycoidodon. The postzygapophyses resemble those of the lumbar vertebra. 

 The thirteenth is the anticlinal vertebra. 



In the first lumbar the anteroposterior diameter of the transverse process is twice that of the 

 last dorsal. The centra of the lumbars are high and narrow, the transverse processes are relatively 

 somewhat shorter than in Merycoidodon, and the neural spines are less forwardly directed. 



In the sacrum the ilium is supported almost wholly by the pleurapophyses of the first sacral. 



The caudals indicate a tail shorter than that of Merycoidodon. The neural arch soon dis- 

 appears, the third caudal having an imperfect canal. The transverse processes, however, continue 

 to be prominent. 



The fourteen ribs on each side indicate a basket quite like that of Merycoidodon. The anterior 

 ribs are heavy and flattened, the middle ribs tend to become more round in cross section, and the 

 posterior ribs decrease in size. The thirteenth and fourteenth lack the tubercular facets. 



The scapula is interesting in showing a very feeble development of the metacromion process, in 

 contrast to that of Merycoidodon. This condition foreshadows the development in later selenodonts 

 and does not argue in favor of an aquatic mode of life. The acromion process, directed downward 

 and forward, is like that of Merycoidodon. The glenoid border is heavy and everted, while the 

 cavity is less in transverse diameter but about the same in anteroposterior diameter as in the earlier 

 form. The coracoid is small and has the characteristic hook-like termination. There is a marked 

 neck. The infraspinous and postspinous fosss are nearly equal, while the subscapular fossa is 

 comparatively small. 



The head of the humerus has a slightly greater diameter anteroposteriorly than transversely 

 and is less convex than in Merycoidodon. The greater tuberosity is also relatively larger, the 

 bicipital groove is more open, and the deltoid ridge is more prominent than in that genus, while the 

 shaft, though much shorter, is very nearly as heavy. The external trochlea is much smaller than 

 in Merycoidodon, and the external border of the anconeal fossa overhangs the pit more than in the 

 latter genus. This humerus shows a trend toward that of modern artiodactyls. 



The external division of the humeral facet of the radius is much reduced, and the medial and 

 internal facets are proportionally larger than in Merycoidodon. The radius and ulna were probably 

 not fused. The shaft becomes quite round and distally turns backward to a greater degree and is 

 flatter transversely than in the Oligocene genus. It is also more outwardly directed, thus indicating 



