74 AMERICAN MESOZOIC MAMMALIA 



of the centroexternal cusp. This intermediate cusp is always well defined and in some 

 specimens is nearly as large as the centroexternal one. 



In exceptionally well-preserved material, such as Y.P.M. No. 13740, it may be 

 seen that the crest from the internal cusp along the anterior edge of the crown runs to 

 the anterior side of the base of the centroexternal cusp, excluding the anteroexternal 

 cusp from the basin of the crown, and that the crest or raised rim along the posterior 

 edge of the crown does not run to the apex of the posteroexternal cusp but terminates in 

 a minute cuspule posterointernal to the latter. 



M^ is known only in a worn condition, but seems to have had the same elements as 

 the other molars. The general crown level is lower and the internal and anteroexternal 

 cusps are larger relative to the other cusps, the latter seemingly being reduced. 



In Y.P.M. No. 13742, the anterior limb of the posterior intermediate cusp is 

 sharply crested, rather than rounded. 



Skull 



The various maxillary fragments referred to this species reveal very little of the 

 cranial osteology. The maxilla is gently incurved above the canine and anterior pre- 

 molar, but posterior to this it is nearly vertical, strong and wall-like. The anterior root 

 of the zygoma is above the last two or three molars. Below this root the alveolar process 

 curves inward around the last molar. 



Y.P.M. 13740: Length M^-* 5.0 



? Herpetairus humilis, new species 



Type.— Y.P.M. No. 13745, left maxilla with M^■^ 



Horizon and Locality. — Morrison formation, Quarry 9, Como Bluff, Wyoming. 



Diagnosis. — Molars with a slight median transverse ridge. 



This specimen is of rather doubtful affinities but it cannot be placed in any estab- 

 lished species and is of sufficient morphological interest to warrant the application of a 

 name. It resembles H. arcuatus in the known features except that a slight rounded 

 ridge is developed in the basin of the crown between the internal and centroexternal 

 cusps. This is perhaps homologous with the median ridge of Melanodon, but its devel- 

 opment is much less than in that genus. 



