A NEW HOPLOPHONEUS FROM THE TITANOTHERIUM 



BEDS. 



Investigation aided by a Grant from the Marsh Fund of the 

 National Academy of Sciences. 



By WILLIAM J. SINCLAIR. 

 (Read April 22, 192 1.) 



Comparatively little prospecting was done by the Princeton 1920 

 expedition to South Dakota in the Titanotherium beds, but as a by- 

 product of some studies on the contact between this formation and 

 the Oreodon beds the specimen described herewith was found, which 

 is interesting both as the oldest representative of the genus Hoplo- 

 phoneus and as a higher type of specialization in the development of 

 the chin flange than is found in the various species of this saber- 

 tooth from the Oreodon beds. If Hoplophoneus mentalis is an- 

 cestral to Ensmilus dakotensis from the Protoceras beds, as seems 

 possible, perhaps we have here an illustration of a case of survival 

 of the fittest adaptation, with dying out of the shorter-chinned 

 mutants of the Oreodon beds, in which less adequate protection was 

 afforded by the chin flange to the long upper saber-teeth. 



Fig. I. Hoplophoneus mentalis. type specimen, side view of the left half 

 of the lower jaw, two thirds natural size. No. 12515. 



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