MORRISON MAMMALS: PANTOTHERIA 51 



Arc/M^ofrigon Simpson 1927 

 1927. Arc/iaeotrigon, Simpson, Amer. Jour. Set. (5) XIII, 410. 



Definition. — Dental formula !< Ci P2 M3.4. Paraconid low but distinct, talonid 

 with low distinct cusp, metaconid intermediate in height. Talonid semicircular in 

 plan. No postcanine diastema, molars not spaced. Horizontal ramus stout. Symphysis 

 short. Coronoid arising immediately posterior to last molar. 



Type. — A. brevimaxdlus Simpson. 



Distribution. — Morrison formation, Wyoming. 



The paurodontids are, on the whole, the least satisfactorily known of the Morrison 

 pantotheres. The specimens are so few in number and often so difficult of interpretation 

 that a taxonomy built on them is purely provisional and subject to radical change if 

 new and more complete material is discovered. Archaeotrigon is rather the best known 

 of all. It is unquestionably close to Paurodon, yet it differs from the type (and only) 

 specimen of the latter genus in characters which must be considered generic. At the 

 same time, the possibility must be distinctly recognized that some of the differences 

 between these two genera may be due to age diflferences or to faulty preservation. It is 

 reasonably certain that the genera are distinct, but the distinctions may be either less 

 or greater than here indicated. Two species are recognized. 



Archaeotrigon brevimaxdlus Simpson 1927 

 1927. A. brevimaxillus, Simpson, Amer. Jour. Sci. (5) XIII, 410. 



Type. — U.S.N.M. No. 2793. Right lower jaw with C, P1.2, and M1.2. 



Referred Specimen. — Y.P.M. No. 13648. Left lower jaw in matrix, internal 

 aspect, with P1.2, and M2-3, badly crushed. 



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



Diagnosis. — Probably only three molars. Paraconid shelf narrow. Talonid rela- 

 tively long. 



Dentition 



In the type specimen the dental formula is apparently I4 Ci P2 M3. The only ques- 

 tion is whether another molar may not have appeared later in life. This seems rather 

 improbable as the specimen seems fully adult and the base of the coronoid is immedi- 

 ately back of Ma, closer to this tooth than the coronoid is to Mt in Paurodon. In the 

 related A. distagnms, however, there are apparently four molars, and the definitive 

 number remains in doubt. 



There are alveoli for four incisors, the first large and procumbent, the following 

 ones progressively smaller and less procumbent. Unlike that of most pantotheres, the 

 incisor series is not a straight anteroposterior line, but curves inward anteriorly. 



The canine is much like that of Paurodon, so far as preserved, but the root has a 

 shallow vertical median depression. The premolars also resemble those of Paurodon, 

 although the internal cingulum, instead of passing into the anterior cutting edge 



