FORT UNION OF CRAZY MOUNTAIN FIELD, MONT. 109 



The lower molars of Gelastops so closely resemble those of Didel- 

 phodus as to make a hypothesis of relationship most reasonable. 

 The only really clear difference is the greater elevation of the trigo- 

 nids in Gelastops and this is not extreme. The upper molar men- 

 tioned below also adds to the evidence, but the reference is not 

 certain. 



Gelastops thus resembles both Acmeodon and Didelphodvs and is 

 structurally almost intermediate between the two. Without pro- 

 ducing absolute proof, it strongly suggests that the three genera 

 Didelphodon, Gelastops, and Acmeodon belong to a natural group, 

 and they are here tentatively recognized as constituting the sub- 

 family Didelphodontinae. There are still difficulties in the way of 

 this interpretation that perhaps can be resolved only by the discovery 

 of upper dentitions of Gelastops and Acmeodon. The outstanding 

 difficulty is that the series Didelphodus-Gelastops-Acmeodon is an 

 excellent structural sequence in that order, on the basis of the parts 

 now known in all, but that the most primitive genus, Didelphodvs, is 

 the latest in time, a relationship that is fully possible but that re- 

 quires better evidence for definitive acceptance. 



GELASTOPS PAROUS Simpson 



Figures 12, 13 



Gelastops parous Simpso?;, 1935d, p. 227. 

 Emperodon acmeodontoides Simpson, 1935d, p. 229. 



Type. — U.S.N.M. no. 6148, right lower jaw with canine. Mi, M3, 

 and alveoli. Collected by A. C. Silberling. 



Type of Emperodon acmeodontoides. — U.S.N.M. no. 9850, right 

 lower jaw with P4, M2, and part of P3. Collected by A. C. Silberling. 



Horizon and locality.- — Type probably from Silberling Quarry,^^ 

 other specimens from Gidley Quarry, Fort Union, Middle Paleocene 

 horizon. Crazy Mountain Field, Mont. 



Diagnosis. — Sole known species of genus. Measurements in table 22. 



Remarks. — Besides the type and that of the synonym, there are 

 two referred specimens from the Gidley Quarry in the National 

 Museum collection: U.S.N.M. no. 9601, a right lower jaw with 

 M2_3 and alveoli, and U.S.N.M. no. 9446, a right lower jaw with M2_3. 



From the alveoli it appears that a reduced Pi was present; this 

 tooth is absent in Acmeodon and unreduced in Didelphodus. V2-3 

 were present and each had two well-separated roots. The structure 

 of P4 has already been sufficiently described except to note that the 

 protoconid tip bends outward (labially) in a peculiar way, approach- 



" There is some inconsistency in the available records. It is certain that the type is from the Fort Union 

 No. 2, and from its preservation it must be from one of these two quarries, but some error or omission had 

 been made in recording the field number. Being collected in 1908, it is more likely to be from the Silberling 

 Quarry. 



