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BULLETIN 16 9, UNITED STATES NATIONAL MUSEUM 



Renewed study has necessitated some modification of the arrange- 

 ment given in a preUminary paper (Simpson, 1935d). In the first 

 place the species there called Chriacus pusiUus was wrongly placed in 

 Chriacus, It belongs in Metachriacus , where it is indistinguishable 

 from the genotype, M. punitor. This makes possible some redefini- 

 tion of Metachriacus, which proves to be even more distinct from 

 Chriacus than was at first supposed. The species called Tricentes 

 latidens (quoted from Dr. Gidiey's notes) is now seen to be distinct 

 from Tricentes, despite a very marked adaptive resemblance, for it has 

 Pi, apparently always absent in Tricentes, and the paraconids are in 

 quite a different position although reduced about as in Tricentes. It is 

 necessary to erect a new genus for this species, since redefinition of 

 Tricentes to include it would make that genus so broad as to destroy 

 all balance in the generic arrangement of the family achieved by 

 Matthew and others. 



Spanoxyodon is a peculiar type with aberrant speciahzation, and 

 Prothryptacodon is evidently a very primitive form slightly but de- 

 finitely modified in the direction of Thryptacodon. 



For convenience in identifying fragmentary material a summary 

 (table 39) of lower tooth dimensions is here given. 



Table 39. — Measurements (in mm) of loiver dentition in seven species of 



Oxyclaeninae ' 



1 The figures in this table are means for the number of specimens (in all, not necessarily for every dimen- 

 sion) shown in parentheses. The range of variation is known reasonably well only in Metachriacus punitor 

 and M. provocator, as given hereinafter. The dimensions in themselves are not necessarily distinctive, for 

 instance between Metachriacus provocator and Spanoxyodon latrunculus, but in such cases there are, of 

 course, well marked nonnumerical distinctions. Some worn and incomplete specimens might, however, 

 be unidentifiable in such cases. 



Genus PROTHRYPTACODON Simpson 



Pr othrT/ptacodon Simpson, 1935d, p. 233. 



Type. — Prothryptacodon jurens Simpson. 



Distribution. — Middle Paleocene, Fort Union, Montana. 



Diagnosis. — Canine semiprocumbent, root extending beneath pre- 

 molars, as in Thryptacodon. Pi -2 spaced widely. P4 similar to 

 Thryptacodon. Molar trigonids higher than in Thryptacodon, para- 



