98 THE MERYCOIDODONT1D/E 



Etymology: Mesoreodon (middle + Oreodon). 



Species: 



M. chelonyx Scott 1893. Genotype. 

 M. intermedins Scott 1 893. 

 M. megalodon Peterson 1906. 

 M. scotti Schlaikjer 1934. 



Mesoreodon chelonyx Scott 1893 

 Fig. 65 h PL IX, fig. 4; PI. XI, figs. 1-2 



Original Reference: The mammals of the Deep River beds. Amer. Nat., XXVII, pp. 659, 661. 



Type Locality: Smith River Valley, Montana. Other specimens collected on Bear Creek Mountain and 

 Hill 66, Goshen County, Wyoming. 



Geologic Horizon: Lower Miocene (lower Harrison). 



Type: Genoholotype, Cat. No. 10425 P.U.M., skull, jaws, and most of skeleton. Collected by O. C. 

 Mortson of the Princeton Expedition of 1891. 



Specific Characters: The size and shape of the skull is much like that of Eporeodon, but 

 the face is somewhat more swollen in the maxillary region. The maximum expansion of the zygo- 

 mata is about midway of the squamosal portion. The zygoma is moderately light and narrow, 

 although the external border is thickened and rugose and the posterior part has a marked upward 

 slope. The malar is of medium depth beneath the orbit. The nasals are posteriorly pointed, of 

 moderate width and transverse convexity, and unreduced. The lacrimal bone is large and the pars 

 facialis nearly square in outline. The lacrimal fossa is well marked but is not so deep as the average 

 depth of this pit in Eporeodon, although this character apparently varies with sex and with the 

 development of the canines, probably being more marked in the males. The frontals are moderately 

 narrow and somewhat vaulted, in general resembling the same bones in Eporeodon. They extend 

 forward about to the anterior edge of the lacrimal. The orbits are closed, circular in shape, and 

 proportionally rather small, and they look mainly sidewise. The temporal ridges unite above the 

 postorbital constriction to form a long, thin, and high sagittal crest, approximately a third of the total 

 skull length. The supraoccipital crest has widespread wings and is produced well beyond the 

 occipital condyles. The brain case is somewhat more expanded than in Aderycoidodon culbertsonii. 

 The external auditory meatus is a long and nearly straight tube, directed outward, upward, and back- 

 ward. The palate is wide and gently convex, with the U-shaped palatonarial border just back of 

 the last molars. The basicranial axis is moderately shallow. The glenoid articular surface is gently 

 convex, and the postglenoid process is low, broad, and very massive. The bulla; are moderately 

 well developed but vary in size to some extent. The paroccipital process is transversely broad at 

 the base, closely applied to the bulla, and its distal part is prismatic in cross section and is slender and 

 tapering. Index: 0.56. 



Mandible: The symphysis is strong and has a tubercle at the base. The symphysiodental angle 

 is about 40°. The chin is concave in profile. The ascending ramus is moderately deep, but with no 

 marked angle, and the posterior edge is rather straight. The coronoid process is slender and short, 

 while the sigmoid notch is wide open and not deep. Transversely the condyle is much extended. 

 The inferior margin of the mandible is gently uparched. 



Foramina: The infraorbitals are above the anterior part of P 4 . 



Dentition: These teeth show an incipient hypsodontism not seen in Eporeodon. Superior: 

 The incisors are small, with simple crowns, anteroposteriorly compressed, and somewhat pointed, 

 and they increase regularly in size from first to third. The canine is of the usual shape and pattern 

 but has a deep groove on the inner face. The premolars are relatively larger than in Eporeodon. 

 P 1 has an anterior and posterior basin, the latter formed by a slight elevation of the cingulum. P 2 



