68 AMERICAN MESOZOIC MAMMALIA 



deep circular notch. The condyle itself is broadly attached and wide transversely. The 

 articular surface faces upward and a little backward and is transversely cylindrical. 

 It is far above the alveolar border. The angular process is much as in other dryolestids, 

 small, styliform, directed backward, continuing the general curve of the lower border. 



When both external and internal aspects are taken into consideration, the horizon- 

 tal ramus is seen to be similar to that of Laolestes, although it is relatively more slen- 

 der. The supposed restriction back of the last molar seems not to be a real original 

 feature of the jaw. 



The internal aspect of the jaw is best seen in U.S.N.M. No. 2142. The pterygoid 

 fossa is broad and shallow, bounded below by a moderately developed crest which 

 becomes obsolete as it passes backward toward the angle. The dental foramen is long 

 and oblique and is situated at the anterior corner of the pterygoid fossa. The internal 

 groove begins just anterior to the dental foramen and falls rapidly toward the lower 

 border, reaching it and disappearing beneath M4. The symphysis ends posteriorly 

 beneath Pi. 



Measurements 



Lengths : 



P3 P4 Ml Mo M3 M4 M5 Me M7 



Y.P.M. 11883 0.75 0.7 0.75 0.85 0.85 0.85 



U.S.N.M. 2412 0.8 0.8 0.7 0.8 . . . . 0.9 



Y.P.M. 13732 .. .. .. .. .. •• 0.85 i.o 0.9 



lin^iQC 2 o 



Y.P.M. No. 13732: Depth of ramus below M.A ", 



U.S.N.M. No. 2693 : M^.g 5-9 



End of Mg to tip of angle 7.6 



Top of coronoid to lower border 6.5 



Amblothermm debilis Simpson 1927 



1881. Dryolestes gracilis, Marsh, Amer. Jour. Sci. (3) XXI, 513. 

 1927. Amblotherium debilis, Simpson, Amer. Jour. Sci. (5) XIII, 411. 



Type. — Y.P.M. No. 1 1821. Left lower jaw with Mi-s and roots or alveoli of other 

 teeth. 



Principal Referred Specimens. — Y.P.M. No. 13728. Right lower jaw in ma- 

 trix, outer aspect, with Mi-e. 



Y.P.M. No. 13730. Posterior part of right lower jaw, in matrix, inner aspect, with 

 M5-7. The outer aspect of Ms-? has also been exposed. 



Y.P.M. No. 13734 and U.S.N.M. No. 2726 are also referred. 



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



Diagnosis. — Molars smaller than in A. gracilis. External cingulum weaker. Pa-^ 

 slightly lower than me"^ on all molars. M7 markedly smaller than Me. 



Marsh's Dryolestes gracilis must be referred to Amblotherium or "Stylacodon" 

 Marsh. It thus becomes a homonym of Amblotherium gracilis {^Stylacodon gracilis 

 Marsh) and a new specific name has necessarily been given it. The distinctions given 



