66 AMERICAN MESOZOIC MAMMALIA 



2. Five premolars. 



3. Constriction behind molar teeth stronger than in Stylacodon. 



4. Jaw deeper below molars. 



5. Lower margin convex, as in Dryolestes. 



None of these characters is valid, i is due to wear and to breakage. 2 was based 

 on inadequate evidence and is surely erroneous. 3 must have been due to incomplete 

 preparation, as there is now no postmolar constriction in either "Stylacodon" or "Lao- 

 don." 4 is due wholly to the fact that the inner rather than the outer aspect of the jaw is 

 presented. As regards 5, it is true that the preservation of different parts, seen in differ- 

 ent aspects, in the two cases gives a different appearance, but the superposition of accu- 

 rate tracings from enlarged photographs shows that there is actually no significant 

 difference in the form of the jaw, or in any other character. Laodon and Stylacodon are 

 certainly synonymous. This American genus cannot now be distinguished from the 

 English Amblotherium-Achyrodon-Trouessartella {"Stylodon") and for the present, 

 at least, all must be referred to Amblotherium, the earliest valid name applied to the 

 genus. This reduction of five generic names to synonymy has resulted also in a reduc- 

 tion of the number of species. Two species, A. soricinum and A. nanum, are recog- 

 nizable in the English Purbeck beds and two, A. gracilis and A. debdis, in the Ameri- 

 can Morrison formation. Stylacodon validus Marsh, as already pointed out, is a syno- 

 nym of Dryolestes -priscus. 



Amblotherium gracilis (Marsh 1879) 



1879, Stylacodon gracilis, Marsh, Amer. Jour. Sci. (3) XVIII, 60. 

 1887. Laodon venustus, Marsh, Amer. Jour. Sci. (3) XXXIII, 337. 

 1927. Amblotherium gracilis, Simpson, Am.er. Jotir. Sci. (5) XIII, 412. 



Type. — Y.P.M. No. 11883. Left lower jaw, in matrix. Anterior fragment, tooth- 

 less, inner aspect. Middle portion, matrix impression only. Posterior portion, outer 

 aspect, with P4, M1-4. P3 preserved separately. 



Type of Laodon venustus. — U.S.N.M. No. 2142. Left lower jaw in matrix, 

 inner aspect, with P3.4 and M1-7. Fig'd, Marsh 1887, PI. IX, fig. 5. 



Principal Referred Specimens. — U.S.N.M. No. 2693. Left lower jaw in ma- 

 trix, outer aspect, with P4 and Mi-s. Fig'd (as Stylacodon gracilis), Marsh 1887, PI. 

 IX, fig. I. 



Y.P.M. No. 13732. Right lower jaw with M5.7. 



Y.P.M. Nos. 13731 and 13733 are also tentatively referred. 



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



Diagnosis. — Length of molar series about 6.0 mm., molars 20-25 P^r cent larger 

 than in A. debilis. External cingulum strong. Pa'' = me'' on Me 7. M7 nearly as large 

 as Mo. 



Dentition 



The four premolars agree in most respects with those of Laolestes, save in their 

 smaller size. Each has a single high somewhat recurved main cusp with a small pos- 



