50 GEOLOGICAL SURVEY OF TEXAS. 



present one, especiallj'^ the canine-molars, of which a good idea may be 

 gained in the M. leptostomus, from the form of its alveolus. 



The palatal portion of the premaxillary bones is bifurcate from an acute 

 angle posteriorly, whicli is opposite the posterior border of the alveolus 

 of the canine molar. Neither moiety extends anterior to the anterior 

 border of that alveolus. The alveolar ridges approach posterior to these 

 alveoli so as to be separated by an interspace equal to two-thirds that 

 which separates the alveoli. In M. jeffersonii and M. leidyi this width is 

 equal to that which separates the canine-molars; and the premaxillary 

 palatal emargination is rounded, broadly so in the M. leidyi. The M. 

 leptostomus is remarkable for the narrowness of the palate throughout, the 

 width equaling about two-thirds that of the corresponding region in the 

 two species mentioned. It is in consequence narrower than the transverse 

 diameter of the molars, while it equals that diameter in the two species 

 already described. 



The form of the canine-molar is dififerent from that characterizing the 

 other known species of the genus. The alveoli, instead of having con- 

 cave internal borders, divergent from each other outward and backward, 

 as in M. jeffersonii and M. leidyi, have their internal borders parallel and 

 directly anteroposterior. They present posteriorly a wide border at right 

 angles to the internal border, and passing into the latter by a rounded 

 angle. The form of the tooth is therefore more or less triangular, or 

 similar to that of a molar tooth placed longitudinally. This is a suspi- 

 cion only, as the external and anterior alveolus walls are unknown. The 

 form of the known part of the alveolus, however, shows that the canine 

 molar is not only very dififerent from that seen in the two species men- 

 tioned, but is equally distinct from those of the M. loxodon, M. wheatleyi 

 and M. tortulus, Cope, and the M. dissimilis of Leidy. Neither the in- 

 ternal nor the posterior alveolar borders display any grooves or concavi- 

 ties, but are perfectly plane. 



The alveoli of the molar teeth indicate that they differ in the antero- 

 posterior diameters of their widest or internal extremity. The supposed 

 last molar is about equal in dimensions to the middle molars of the M. 

 jeffe)-sonii and M. leidyi, and is one-fourth larger than those of M. wheat- 

 leyi and M. dissimilis. Its diameters are uniform from base to crown. 

 Its horizontal section is a narrow triangle with rounded angles. Its an- 

 teroposterior diameter is smaller in relation to its transverse than in any 

 species known to me. The forms of the inner borders of the alveoli indi- 

 cate that some of the other molars are relatively wider in the fore and aft 

 direction. 



