MESOREODON 97 



Foramina: The small infraorbitals are located above the anterior part of P 3 . The supraorbitals 

 are 15 mm. apart, that is, moderately close to the sagittal suture, and they have well-defined grooves 

 extending as far anterior as do the frontals. The posterior palatines are opposite P . 



Dentition: The superior incisive border is nearly transverse. The teeth are very much worn, 

 and M 1 , M 3 , and several of the premolars have been lost. The incisors are small, with roots 

 circular in section. The canine is large and has a short posterior diastema. P 4 is small. M" is 

 wider than long and has no internal cingula, except between the lobes, but there is a trace of an 

 anterior cingulum. The molar-premolar index is approximately 0.88. 



Discussion: In the Marsh Collection this species is represented by a comparatively few skulls, 

 all from either Turtle Cove or Bridge Creek. These specimens differ, however, from the type in 

 certain aspects, though not sufficiently to invalidate the identification. 



MESOREODON-PRONOMOTHERIUM STOCK 



Genus MESOREODON Scott 1893 

 Table 5 



Original Reference: The mammals of the Deep River beds. Amer. Nat., XXVII, p. 661. 



Genotype: M. chelonyx Scott 1893. 



Genoholotype: Cat. No. 10425 P.U.M., skull, jaws, and the major part of the skeleton of one individual. 



Distinguishing Characters: Skull patterned in general after that of Eporeodon, meso- 

 ccphalic to brachycephalic (indices 0.56-0.74), and from 248 mm. to 270 mm. in length; nasal 

 bones long and unreduced; premaxillaries well developed; face, with orbits, about half length of 

 skull, and muzzle somewhat swollen; brain case moderately small; antorbital fossa larger but 

 shallower than in Eporeodon; orbits closed and small to medium in size; frontals rather narrow 

 but not reduced; malar moderately deep and zygomatic arches light to medium; bulke mediumly 

 large; mandible medium to fairly deep posteriorly and symphysis strong; infraorbital foramen above 

 middle of P 3 to anterior of P 4 . 



Dentition: Shows incipient hypsodontism; otherwise teeth resemble those of Eporeodon in 

 most respects. They differ in the relatively larger size of the upper premolars. P 3 reduced in 

 anterior part, with a small anterointermediate crest and posterior basin. The parastyles and meso- 

 styles are thin and compressed, and M 2 and M 3 are more nearly equal in length. P 3 , after part of 

 posterior crescent only developed and joined with posterointermediate crest. P 4 , posterior crescent 

 confluent with pillar-like cusp at junction of that crescent and median crest. Superior molar-pre- 

 molar indices 0.81-0.90 and inferior 0.71-0.84. 



Discussion: Mesoreodon is not readily distinguishable from Eporeodon except in its slightly 

 more progressive characters. Scott in fact considered that it might be ranked as a subgenus of the 

 latter. There are, however, distinctions which seem to entitle this form to generic value. 



My opinion is that Mesoreodon holds an intermediate position between Eporeodon on the one 

 side and Promerycochcerus and Merychyus on the other, and yet the known species of Mesoreodon 

 are more or less contemporaneous with these three genera. The skeleton of M, chelonyx is inter- 

 mediate between that of Eporeodon and Promerycochaerus, while that of M. intermedins shows a 

 decided tendency in the direction of Merychyus. 



This genus must have branched off from the Eporeodon stem stock sometime in the upper 

 Oligocene and continued along in a more conservative manner than its contemporaries. 



