COPE.] PALEONTOLOGY LOUP FOEK EPOCH. 523 



to throw the inner and median posterior areas together. The anterior 

 median is well isolated and subrouud. There are no folds of the enamel- 

 plates whatever. 



Measurements. 



M. 



Length of No. 1 from roots 0.027 



Width of autero-posterior 019 



Width of extero-interior 020 



Length of No. 2 032 



Width of autero-posterior 021 



Width of extero-internal 018 



Length of No. 3 018 



Width of autero-posterior 021 



Width of extero-interior , 022 



From the neighborhood of the Pawnee Buttes, Colorado. 



PROTOHIPPUS, Leidy. 



Peotohippus labrosus, Cope. 



Having obtained a number of fragmentary and entire crania refer- 

 able to species of the present genus, it becomes possible to correlate 

 the mandibular with the maxillary forms, dentition, &c., as it has not 

 been possible to do heretofore. Of mandibles there are four types, 

 which I refer to species as follows : 



Symphysis flat, shallow ; no diastema between their incisor and canine 



teeth. — P. lahrosus. 

 Symphysis narrower, deep ; inferior molars smaller. — P. sejunctus. 

 Symphysis narrow, deep, contracted, and smaller ; lower molars 

 larger. — P. perditus. 



These comparisons are instituted on one mandible of the first ; two 

 entire and three incomplete ones of the second; and two of the third 

 types, all but two accompanied by superior molars or crania. The 

 specimen of P. lahrosus embraces the right maxillary bone, containing 

 five molars ; a second specimen includes three superior molars of the 

 left side ,• it is also represented by several isolated molars. 



Frotohippus labrosus lesembles the two species described by Leidy as 

 Merychipptis in the short crowns and long roots of the molar teeth, with 

 thickened external ridges, separated by thin bauds of cementum. It 

 therefore differs from Frotohippus perditus and P. placidus, resembling 

 the first named in size. It is exactly intermediate between the P. insignis 

 and P. mirabilis in size, and to it is no doubt to be referred Dr. Leidy's 

 No. 4 of the latter.* Either there are three species of the present char- 

 acter, or Dr. Leidy's and the present forms must be arranged under one 

 appellation. I prefer retaining them as distinct for the present, since I 

 have nearly identical measurements in six different individuals, and 

 four of the F. perditus equally uuiform in dimensions. The latter always 

 slightly exceed those of the P. labrosus, and differ in the longer dental 

 crowns, with subacute exterior ridges ; typically, the internal columns 

 are oval in section, but may occasionally be subcyliudric ; they are 

 cylindric in P. labrosus. The first specimen above mentioned I regard 

 as typical, and describe it as follows : 



The first premolar is well developed ; in the first molar, the anterior 

 lake is isolated from the inner fold. The anterior inner column is cylin- 

 dric in all the teeth ; the posterior similar, but joined with its crescent by 

 attrition in most of them. The boundaries of the crescents are all sim- 

 ple, except a tendency to the middle infolding of the adjacent borders 



* Ancieut Fauna of Dakota and Nebraska, p. 300, figured plate xvii, Figs. d-9. 



