MERYCHYUS 



233 



process invades it to a point below the last quarter of the orbit. The maximum expansion is just ahead 

 of the glenoid surfaces. The facial vacuities are of moderate size and are bounded by the lacrimal, 

 maxillary, and frontal bones. Their maximum diameter is at about a 45° angle with the tooth row. 

 The nasal bones are bluntly rounded, nearly truncated posteriorly, with their greatest width slightly 

 back of the narial opening. In advance of that point they diminish rapidly in width, terminating in 

 a rounded border; posteriorly they decrease very gradually in width. They are moderately flat 

 posteriorly, becoming more convex anteriorly. The lacrimal bone is large, with a considerable area 

 on the face, and the lacrimal fossa is moderately large. The frontals are rather wide and gently 

 convex in both directions. The orbits are nearly round, about normal in size, and not elevated. The 

 temporal ridges unite well aft over the posterior of the glenoid surfaces to form a short sagittal 

 crest. The supraoccipital crest overhangs the condyles to some extent. The brain case is moder- 

 ately expanded. The external auditory meatus is directed outward and backward. The palate is 



VM.I3774 



Fig. 169. — Merychyus siouxensis Loomis. Skull. HT. Cat. No. 13774 A.M.N.H. 1/2 nat. size. 



(After Loomis, 1924.) 



wide, and the U-shaped palatonarial border lies slightly behind the posterior of M 3 . The basicranial 

 axis is not steeply inclined. The glenoid surface is nearly flat. The postglenoid processes are 

 roughly triangular in basal outline, with the apex inward and nearly touching the bulls. The latter 

 are much inflated and extend anteroposteriorly from the paroccipital processes to a line along the 

 middle of the glenoid surfaces. The paroccipital processes are slender and triangular in outline, 

 extending not far below the inferior surfaces of the bulla;. Index: 0.57. 



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

 1 7 mm. apart, or fairly close to the mid-line, with grooves leading forward to the suture between the 

 nasal and frontal bones. 



Dentition: The incisors are separated by short diastemata from each other and from the canine. 

 There is also a diastema between C and P 1 . The premolars are not especially crowded, although P 1 

 and P 2 overlap. Loomis (p. 33) says that: "The anterior portion of premolars 2 and 3 is (for 

 this genus) long, and the anterior basin is divided into two parts." The molars are subhypsodont. 



Discussion: This species is so close to M. elegans that the resemblance is striking, thus showing 

 the conservatism of this genus throughout the Miocene. However, it is more primitive than the 

 upper Miocene form, for example in the teeth, and may well have been ancestral to the genotype. 

 The total and individual tooth measurements are almost exactly the same in both, with the index 

 (m.-pm.) alike, that is, 0.71. The incisors are not spaced, nor is there a noticeable diastema between 

 I 3 and C in the genotype. These differences, together with those in the premolars noted above, 

 constitute the major distinctions between the two species, as now known. Unfortunately we know 

 little of the skull of M. elegans, but I believe that in M. siouxensis we have the nearest approach to 

 the former species of any so far described and that it may well stand directly ancestral to it. 



