52 AMERICAN MESOZOIC MAMMALIA 



gently, meets the latter at an angle. The first two molars and alveoli for the third are 

 preserved. M2 w^as the largest of the three. The molars diflEer sharply from Paurodon 

 in the better development of the internal cusps. The pa'' is still small and low, but it is 

 a well-defined cusp and not a mere sloping shelf. The mc* is larger and better defined 

 than in Paurodon, and the heel is longer and bears a small but true cusp. 



Mandible 



The horizontal ramus is smaller than that of Paurodon but of similar proportions. 

 There are two mental foramina, low down on the mandible, one beneath the canine and 

 one beneath Pi. The coronoid arises at about 30° immediately posterior to M3. The 

 internal groove is small and narrow. 



Measurements of Type 



Lengths : Depth of ramus 



C Pi P2 Ml Mo below M2 



i.o 0.9 1.2 1.2 1.6 3.4 



Archaeotrigon distagmus^ new species 



Type. — Y.P.M. No. 13641. Right lower jaw with M2-3- 



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



Diagnosis. — Probably four molars. Paraconid shelf wider. Talonid shorter. 



Dentition 



Imperfect as is the material on which the present species is based, it is of vital 

 importance for the study of the paurodontids. Two molars are present and these are 

 followed by alveoli for another, much smaller, molar. Two interpretations are possible, 

 both accompanied by various difficulties. If the two molars are M1-2, then Mi is larger 

 than M2, a unique character among pantotheres, and M3 is probably relatively more 

 reduced than in A. brevimaxUlus. If, as is more probable, these teeth are M2-3, then 

 there were four molars whereas it is probable that the genotype had only three. Provi- 

 sionally adopting the latter alternative, the significance of the added molar is doubt- 

 ful. It may (improbably) be due only to greater individual age, or to an individual 

 anomaly, or to taxonomic distinction. Provisionally, it is considered as one of the 

 characters distinguishing the species. Except for the fact that the paraconid shelf is 

 wider, springing distinctly inward as well as forward, and that the heel is somewhat 

 narrower, the molar structure is as in A. brevimaxUlus. 



Mandible 

 The mandible is stouter than in the genotype and the internal groove is higher. 



Measurements 



Length Mg 1.5 mm. 



Length M 3 1.3 mm. 



Depth of ramus below Mo 3.6 mm. 



