MERYCHIPPUS. 



in: 



Merychippus isonesus quartus mut. nov. 

 Plates 41.3, 45.4, 46.3, 52.3. 



Horizon and locality. — Sheep Creek phase of Arikaree formation, western Nebraska, lower Middle Miocene. Type 

 collected by W. D. Matthew of American Museum expedition of 1908. 



Type. — Amer. Mus. 14184. Part of skeleton without skull parts or teeth. 

 Type figure — Plates 41.3, 45.4, 46.3, 52.3. 



Characters. — (1) Limb bones as long as in il/. isonesus quintus but decidedly more slender; (2) shafts of limb bones of 

 same weight as in il/. iso?icsns tertius but considerably more elongate; (3) metapodials of same length as in il/. isonesus 

 quintus but smaller at distal ends; (4) shafts of lateral digits and the lateral phalanges much as in il/. isonesus secundus. 



An intermediate premutation stage is indicated by this type. The limb bones indicate, however, that it is to be 

 regarded at present as a definable type. 



Merychippus isonesus quintus mut. nov. 

 Plates 13.7,7a, 14.1, 41.4, 45.2,5, 46.4. Text Fig. 81. 



Fig. 81. Merychippus isonesus quintus Osborn, Amer. Mus. 14185. (1) Side view of left premolar-molar series of 

 the type; (7) crown view of same; (7a) side view of m 3 of the type. All figures natural size. 



Horizon and locality. — Sheep Creek phase of Arikaree formation, western Nebraska, lower Middle Miocene. Type 

 collected by W. D. Matthew of American Museum Expedition of 1908. 



Type. — Amer. Mus. 14185. Upper and lower dentition and a large part of a skeleton. 

 Type figure — Plates 13.7,7a, 14.1, 41.4, 45.2,5, 46.4, text Fig. 81 of this Memoir. 



Characters. — (Matthew, 1913, Osborn, 1918) (1) The type a larger animal than any of the preceding premutations, 

 approaching M. seversus in the plications of the enamel; (2) crochet bifid, i. e. with a large spur projecting into the pre- 

 fossette and a large inferior pli caballin fold; (3) protocone separate from protoconule in little worn molars, uniting with 



