14 



acter, orDr. Leidy's and the present fprms must be arranged under one 

 appellation. I prefer retaining them as distinct for the i)resent, since I 

 have nearly identical measurements in six different individnals and four 

 of the P. perditits equally uniform in dimensions. The latter always 

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

 crowns, with subacute exterior ridges ; typically the internal columns 

 are oval in section but may occasionally be subcylindric ; they are 

 cylindric in P. lahrosus. 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 

 cylindric in all the teeth; the posterior similar, but joint with its cres- 

 cent by attrition in most of them. The boundaries of the crescents are 

 all simple, except a tendency to the middle infolding of the adjacent 

 borders of the crescents. The teeth are but little curved, and the base 

 of the crown, with termination of the broad longitudinal gutters, is 

 visible, although the attrition of the teeth, especially of the inferior 

 incisors, does not indicate advanced age. 



The mandible is distinguished by the length of the diastema and the 

 flatness and shallowness of the symphysis. The permanent molars are 

 all present in the specimen, and are robust in form. Except in the first 

 and last, they are characterized by the small development of the anterior 

 crescent horn and posterior tubercle of the inner side of the crown. The 

 horn of the anterior crescent of the first molar is well produced inwardly, 

 broad and simple. The entire tooth is narrower than the other molars, 

 except the fifth and sixth. The latter is a little longer than the others, 

 and possesses a posterior crescent smaller than the others. The canines 

 issue from their alveoli very close to the third incisors. The two pairs 

 of first and second incisors are in a nearly transverse line, in consequence 

 of the flatness of the symphysis. The median lake is half worn out in 

 the second incisors. 



Measuremerits. 



M. 



Length four premolars and one molar .087 



Length crown, first premolar .008 



Length crown, second premolar 022 



Width crown, second premolar 019 



Length crown, first molar 018 



Width crown, first molar . - 022 



Height crown, first molar Oil 



Length six inferior molars 113 



Length first inferior molar 020 



Width first inferior molar, (medially) , 009 



Length second inferior molar 019 



Width second inferior molar ^ 012 



Width sy mphyseal trough, (least) 015 



Depth in front forame7i mentale 106 



Expanse of two .middle pairs of incisors 041 



About the size of the ass. 



This species is readily distinguished from the more common P. j>er- 

 ditus by the peculiar forui of the symphysis, more simple molar teeth 

 with shorter crowns, and the constantly smaller size; four mandibular 

 teeth of the latter occupying the same space as five mandibulars in the 

 P. lahrosus. The first premolars are also larger and two-rooted ; those 

 of P. ijerdltus, in three specimens before me, and of P. sejunctus, in one 

 example, being but one-rooted. 



