MERYCHIPPUS. 



105 



Merychippus isonesus secundus mut. nov. 



Plates 13.4, 41.1, 45.1,3, 46.1, 52.1. Text Fig. 79. 



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

 collected by Harold J. Cook of American Museum Expedition of 1908. 



Type. — Amer. Mus. 14179. Fragments of upper and lower jaws with most of the teeth and parts of skeleton. 

 Type figure — Plates 13.4, 41.1, 45.1, 3, 4(3.1, 52.1, text Fig. 79 of this Memoir. 



Characters. — (Matthew, 1913, Osborn, 1918) (1) Teeth the same size as M. iso7icsus. (2) Crochet forked instead of 

 single as in M. isonesus primus; (3) not united to protoconule except in well worn teeth; (4) protocone bulbous, transversely 

 expanded, united to protoconule in half worn teeth; (5) a beginning of the pli caballin, namely, lower branch of crochet; 

 (6) two folds extending from metaloph into prefossette and two folds extending into postfossette; (7) hypostyle slightly 

 plicate. (8) Shafts of limb bones slender; (9) metapodials long and slender, lateral digits considerably reduced; (10) Mts. 

 Ill with strongly compressed shaft; (11) ulnar shaft closely compressed but not united with radius; (12) shafts of lateral 

 digits and lateral phalanges much reduced. 



a/o 7~yP e Cp.rey.) 



4 



Merychippus isonesus secundus 



Fig. 79. Merychippus isonesus secundus Osborn, Amer. Mus. 14179, p 3 -m 2 of the type, crown view. Partly reversed 

 in drawing from right and left sides. Natural size. 



Merychippus isonesus tertius mut. nov. 



Plates 13.5,6, 14.3, 41.2, 46.2, 52.2. Text Fig. SO. 



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

 Type collected by Albert Thomson of American Museum Expedition of 1908. 



Type. — Amer. Mus. 14180. Palate and complete series of grinding teeth, p 2 -m 3 , lower jaws and parts of skeleton. 

 Topotype. — Upper and lower jaws with well worn teeth, Amer. Mus. 14183. 

 Type figure — Plates 13.5, 14.3, 41.2, 46.2, 52.2, text Fig. 80. 



Characters — (Matthew, 1913, Osborn, 1918) (1) This type is slightly larger than the preceding. (2) Crochet uniting 

 with protoconule in half worn teeth, giving off a prominent internal fold, or pli caballin; (3) protocone a long oval, uniting 

 by a strong interior spur with middle compression of protoconule crescent in half worn teeth; (4) prominent plications 

 of the metaloph entering pre- and postfossettes; (5) rudiments of the second plication. (6) Limb bones as in M. isonesus 

 secundus, but shafts more robust than in the preceding type; (7) shafts of lateral digits as in M. isonesus secundus; (8) ulna 

 fused for a short distance with radius. This type is separable from M. isonesus secundus in the antero-posterior elongation 

 of the protocone. It is a premutation which is closely united to the preceding but shows a slight advance in the pli caballin, 

 in the early union of protoconule and crochet, and the somewhat greater anteroposterior elongation of the protocone. 

 The referred specimen, x\mer. Mus, 14183, a topotype, exhibits the manner in which all the enamel plications and folds 

 disappear in much worn crowns. 



