HYRACODONS FROM THE BIG BADLANDS OF SOUTH 



DAKOTA. 



Investigation aided by a grant from the Marsh Fund of the National 

 Academy of Sciences. 



By WILLIAM J. SINCLAIR. 



(Read April 22, 1922.) 



Introduction. 

 Studies in progress at Princeton on the MesoJiippus bairdii- 

 Orcodon adbcrtsoni zone of the White River Oligocene, typically 

 developed in the Big Badlands of South Dakota, have made necessary 

 an examination of specific characters in the genus Hyracodon, con- 

 sidered as an index fossil, and also a review, in this connection, of a 

 recent paper on the subject by Mr. Troxell.^ 



The Four Specific Types. 



Four distinct types can easily be recognized as follows : 

 A. With the end of the protoloph curving round the end of the 

 metaloph in p* and completely fusing therewith in worn teeth, iso- 

 lating a central depression. Inner wall of tooth not deeply grooved. 

 Anterior cross-crest in p^ present, but may be small and little more 

 prominent than the cingulum. Fig. i. 



B. With the transverse valley of p^ blocked by a spur from the 

 protoloph which abuts against the anterior wall of the metaloph. 

 The latter cross-crest is longer than the former in unworn teeth, but 

 tends to shorten up as the tooth wears. Inner wall of tooth deeply 

 indented; p^ with anterior cross-crest present or absent. Fig. 2, 

 A-C. 



C. Transverse valley of p* wide open ; p^ with anterior cross-crest 

 small or absent. Fig. 3, A, B. 



D. Transverse valley of both p* and p^ wide open. Anterior 

 cross-crest in p^ of variable size. Figs. 4, 5. 



1 " New Species of Hyracodon," American Journal of Science, II., July, 

 1921, pp. 34-40. 



65 



PROC. AMER. PHIL. SOC, VOL. LXI, E, AUG. 29, I922. 



