MESOREODON 99 



is larger than P l , with the major and minor anterior basins and with a posterior basin deeper than 

 the corresponding one in P 1 . In P 3 the anterior part is reduced but has a well-developed, though 

 small, anterointermediate crest. The posterior crescent encloses a basin. In the first three premolars 

 the posterior part is the wider, as it is in Eforeodon. P 4 has an anteroexternal pit, which is greatly 

 reduced or absent in M. megalodon. In the molars the parastyles and mesostyles are moderately 

 thin and compressed. M 2 and M 3 are more nearly of a length than are M 1 and M 2 . M 3 is slightly 

 obliquely placed. The molar-premolar index varies from 0.87 to 0.90. 



Inferior: The incisors are smaller than in Eforeodon. They are all about equal in size and 

 strongly procumbent, as is likewise the canine. Pi varies with sex in its size and development. In 

 P 3 , only the after part of the posterior crescent is developed and is united with the posterointermediate 

 crest. P 4 has anterior and posterior valleys but no intermediate crests and has a well-developed 

 posterior crescent, which is confluent with a pillar-like cusp at the junction of this crescent with the 



Fig. 65. — Mesoreodon chelonyx Scott. Left superior dentition. GHT. Cat. No. 10425 P.U.M. Nat. size. 



(Redrawn from Scott, 1895.) 



median crest. This pillar-like cusp is not prolonged so far forward as it is in Eforeodon, nor is it 

 so isolated as in Promerycoc/icerus. The molars are similar to those in Eforeodon. The molar- 

 premolar index ranges from 0.71 to 0.81. 



Hyoid affaratus (PI. XI, Fig. 2): Scott (1895, p. 130) has described this apparatus, so rarely 

 preserved in fossil mammals, and I quote freely from his work. The tympanohyal is a short, stout, 

 cylindrical bar, which is inserted into a depression upon the outer side of the auditory bulla. The 

 stylohyal forms a long, anteroposteriorly broad, but thin and very much compressed bar, which 

 expands at the proximal end. In Merycoidodon this bone is very similar in shape but has the dorsal 

 border slightly thicker than the ventral and both ends are expanded. The epihyal in Mesoreodon 

 is well ossified, is narrower and somewhat thicker than the stylohyal, and tapers distally. In Mery- 

 coidodon it is short and laterally compressed, attached to the upper half of the distal end of the 

 stylohyal, and directed downward at a greater angle than the larger bone. The ceratohyal has a 

 curious paddle-like shape, is slender and rounded where it joins the epihyal, and expands into a 

 rounded blade posteriorly, where it is in contact (not coossified) with the basihyal. The ceratohyal 

 in Merycoidodon is unknown. The basihyal is narrow, depressed, thin, and backwardly curved. 

 It has a glossohyal process on the mid-line of the posterior border. The same bone in Mery- 

 coidodon is a short, transverse, vertically compressed bar. The upper surface is concave and 

 medially smooth and has at each end a tubercle or elevated facet for articulation with the ceratohyal. 

 The lower surface is flattened and very slightly roughened. The posterior border is concave and 

 smooth medially, while from each side the thyrohyals branch off, diverging posteriorly. The anterior 

 border bears the lingual process, which is roughly diamond-shaped in cross section. The inferior 

 and superior surfaces are convex, but the latter has a sharp medial ridge. This process is short and 

 terminates anteriorly in a rather blunt point. The basihyal, the lingual process, and the thyrohyals 

 are ankylosed, so that the shape of this unit resembles a short-handled, two-tined fork. In Meso- 

 reodon the thyrohyals are coossified with the basihyal to form slight, club-shaped tuberosities. 

 These bones are slender, rounded, slightly arched anteriorly, and nearly as long as the stylohyals 

 but are of an altogether different shape. In Merycoidodon the thyrohyals are laterally compressed, 



