268 Annals of the Carnegie Museum. 



prove to be new. The horses, then will probably not form an excep- 

 tion to the general rule. A preliminary study has also been made of 

 the camels and they tell the same story. 



It seems, then, from the evidence thus far obtained, that the two 

 regions (that of Montana and of the plains) all through Oligocene 

 and Miocene times (at least the portions of them represented by fossil- 

 bearing deposits) have been faunally distinct ; or else the preservation 

 of mammalian remains seldom, if ever, exactly coincided in time in 

 in the two localities. 



Mesohippus portentus sp. nov. 

 (Plate LXV, figures 1-4.) 



(Type No. 1622, Carnegie Museum Catalogue of Vertebrate Fos- 

 sils; PL LXV, figs. 2 and 3.) 



The type is a second right upper molar tooth from the Lower White 

 River (" Titanotherium ") beds near Pipestone Creek in Montana. 

 The specimens which I have associated with the type are No. 1624, a 

 left upper premolar; No. 1623, a last left upper molar ; No. 1633, 

 two lower molars ; and No. 1634, one lower molar. All are from the 

 same region and formation as the type. 



Description of Type. 



(1) Size large and (2) crests of molars high for a horse from this 

 horizon, (3) ectoloph very oblique, (4) protoloph and metaloph 

 nearly equal in length, (5) protoloph large and connected with the 

 parastyle, (6) metaloph narrow and nearly connected with ectoloph, 

 (7) protoconule easily distinguishable, but (8) metaconule absent, 

 (9) a crotchet present on the metaloph, and (10) a small conule in 

 the posterior valley of the tooth, (n) a rudiment of a cingulum 

 between protocone and hypocone, (12) parastyle and (13) hypostyle 

 small. 



The metaloph, external to the metacone, is thin and has a sharp 

 crest. The crotchet extends forward and slightly outward from the 

 metaloph where the latter bends outward toward the ectoloph. It 

 extends nearly across the valley to the posterior base of the proto- 

 conule. If the crotchet were higher and united with the protoconule 

 it would produce an "enamel-lake " like those which occur in some 

 Upper Miocene horses. There is a minute conule in the posterior 



