PLATE 18 



CONSTRUCTION OF TEETH IN MERYCHIPPUS. 



COMPARATIVE VIEWS OF TYPICAL UPPER PREMOLAR, P 4 , AND LOWER MOLAR, M 2 , IN DIFFERENT SPECIES. FROM TEETH PRE 



FORMED IN THE JAW BEFORE ERUPTION AND DEPOSITION OF CEMENT. 



All figures natural size. 



Left column. 



Pawnee Creek, Col. Middle Miocene 1. Merychippus paniensis Cope, referred specimen, Am. Mus. 9422. c, Anterior 



view, b, internal view. 

 2. M. calamarius Cope, referred specimen, Am. Mus. 14014. a, External view, 

 b, internal view, c, anterior view, d, posterior view. 



Snake Creek, Sioux ?Lower Pliocene 

 Co., Nebr. 



Sheep Creek, Neb. Middle Miocene 

 Pawnee Creek, Col. 



U U It U U 



Snake Creek, Sioux Lower Pliocene 

 Co., Nebr. 



Right column. 



3. M. isonesus primus Osborn, type, Am. Mus. 14187 (reversed in drawing). 



4. M. sphenodus Cope, referred specimen, Am. Mus. 9408. 



5. M. paniensis, referred specimen, Am. Mus. 8255. 



6. M. calamarius Cope, referred specimen, Am. Mus. 13901a. a, External 



view, b, internal view, c, anterior view, d, posterior view. 



Note the marked progression in the length of the molar crowns and the size of the molar teeth exhibited in the 

 lower grinding tooth of M. isonesus primus Osborn (Fig. 3) and the corresponding tooth of M. calamarius Cope (Fig. 6). 



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