180 THE MERYCOIDODONTID^E 



moderately vaulted. The basicranial axis is very steep. The glenoid surface is apparently rather 

 flat. The paroccipital processes are triangular in section, with the greatest diameter transverse. 

 They abut against the bulla; for two-thirds of their length and descend outward, forward, and down- 

 ward beyond the inferior border of the bullae. These bulla; are apparently unique in shape, being 

 wide and long at the base and sloping from side to side downward to meet in a median antero- 

 posterior^ directed ridge. Index: 0.64. 



Foramina: The infraorbital is double on both sides, the smaller above and in advance of the 

 larger and both above P 3 . The supraorbital foramina are but 1 2 mm. apart. The foramen ovale 

 is of moderate size, while a tiny foramen rotundum is present. The posterior palatines are opposite 

 the interval between P 3 and P 4 . 



Dentition: The molar-premolar index is 0.86. The incisors are rather small, the canines large 

 and strong, and the premolars neither crowded nor set obliquely. The dentition is brachyodont. The 

 posterior crescent of P 3 is thick, and the molars M 2 and M 3 are nearer of a size than are M 1 and 

 M 2 . The metacone of M 3 is directed inwardly 30° from the plane of the outer face of the paracone. 

 All the teeth present are markedly offset from the ones preceding. In general construction these 

 teeth are nearest to those of Merycoides and have decided Ticholeftus affinities. 



Genus PHENACOCCELUS Peterson 1906 

 Table 1 



Original Reference: The Miocene beds of western Nebraska and eastern Wyoming and their verte- 

 brate faunas. Ann. Carnegie Mus., IV, pp. 29-31, figs. 4-5. 

 Genotype: Phenacoceelus tyfus Peterson. 

 Genoholotype: Cat. No. 1263 CM., major portion of skeleton. 



Distinguishing Characters: Size about that of Merycoidodon culbertsonti; skull meso- 

 cephalic, with moderately large facial vacuities and narrow and elongate frontal vacuities ; deep pit 

 in the rear of each occipital bone; premaxillaries somewhat reduced; nasals unreduced; frontals 

 long and narrow; bulla; large; zygomatic arches light; dentition similar to, but less advanced than, 

 that of Merycoides; skeleton not specialized, but with four toes fore and aft and with the pes shorter 

 than the manus. 



Peterson (1928B, p. 163), in comparing this genus with Merycoidodon, concludes: 



Phenacoceelus resembles Merycoidodon more or less closely, throughout the entire skeletal structure. The 

 two genera are alike, (1) in having the dentition numerically as well as in general form, quite similar; (2) in 

 having a short facial region and an elongated cranium (cranium especially long in Phenacoceelus) ; (3) by hav- 

 ing deep lachrymal pits; (4) by having overhanging nasals; (5) by the location of the infraorbital foramen 

 above P 3 ; (6) by having the temporal ridges of similar shape and quite alike in the degree of development; (7) 

 by a general similarity of the detailed structure of the vertebral column; (8) by a similarly large thoracic cavity; 

 (9) by a similarity in most parts of the structure of the limbs; (10) by a general similarity of the manus (except 

 the absence of the pollex in Phenacoceelus). 



Phenacoceelus differs from Merycoidodon culbertsoni: ( 1 ) in having the skull proportionally broader and 

 shorter; (2) in having the orbits directed more upward; (3) by the presence of fronto-nasal vacuities; (4) by 

 the enlargement of the lachrymal pits and the indication of facial vacuities; (5) by having deep pits on posterior 

 face of occipital plate; (6) by the hypsodont cheek-dentition; (7) by having the tympanic bulla of very large 

 size and extending much below the glenoid process; (8) by the feebly developed and low sagittal crest; (9) by 

 the less transverse expanse of the transverse process of the atlas; (10) by the greater vertical and smaller trans- 

 verse diameters of the neural spines of the anterior dorsals; (11) by having one more dorsal vertebra, and one less 

 lumbar vertebra; (12) by having the carpus proportionally and actually higher; (13) by having the tibia shorter, 

 heavier, and the shaft of somewhat different shape; (14) by having the pes relatively shorter and broader. 



