PAL^OSYOPS LEIDY, AND ITS ALLIES. 
371 
behind the post-tympanics, which I take to he a mastoid exposure. A larfre 
exposure of the mastoid may also be inferred from the wide separation of the 
auditory processes. There is no foramen pi’esent at the junction t)f the mastoid 
with the exoccipitals. The petrous bone is preserved and is situated deeply in the 
recess between the glenoid and poshtympanics ; it is placed on a line with the 
glenoid cavity and not as far removed from the foramen ovale as is usually the cast* 
in the larger species. The foramen ovale, although filled with matrix, is plainly to 
be seen ; it is situated on a line with the internal edge of the glenoid facet. 
Relationship and Descent. 
In treating of the relationship and evolution of the genera and s]K*eies in this 
subfamily, I jiropose to confine myself to those forms which 1 have investigated 
as thoroughly as the present known material will allow. It appears to me that the 
relationship of some of the Wind River forms to those from the Dridger pn)|H*r 
is rather uncertain, although I consider that in Palceosyops borealis we have 
a direct forerunner of the Bi’idger sjiecies of this genus. Latubdothcrium is the 
earliest member of this group and it appears that it may have lM*en the ancestor of 
the whole line. The details of the molars in Lambdothcriuin, and especially of its 
premolars, dei^art considerably from those of P. borealis and in fact the latter 
species is much more closely^ related to Palceosyops and Pebuatotlicriu>n. than t«» 
Lambdotherijtm. At any rate it is probable that in an earlier formation than the 
Wind River Eocene, a common form gave origin to the genera Lambdolltennm and 
Palceosyops and I am inclined to believe that the former genus may lx* a side line, 
not leading directly to Palceosyops -a?, by ('ope. and that the latter gt'inis 
has not branched dS. ixom Lambdotherium m Middle Eocene times. 'I'his view is 
supported by the fact that I have lately discovered material in the l’rin<*eton 
collection from the bottom of the Eocene, namely the Wasatch, which is n-ferahle 
to Lambda theriuni^ I have found it rather dillicult to decide which of the two 
genera, Palceosyops or Limnohyops, is the most primitive in its dentition, although 
after considering all their characters and having compared them with nu»re primitive 
types, I believe that Limnohyops is more primitive in its dental structuri's than 
Palceosyops, although in some of the characters of its apiiendicuhir ski'leton. the 
former genus is more specialized than the latter. There is no «pu*stion as to the 
phylogenetic position of the genus Telmatotherhmi in the series. It ci*rtainly is the 
most specialized genus of the group and represents the direct ancestor in the Hridger 
of the more highly specialized genus Diplacodoji. In describing the «*vohitionary 
stages of the species of the Palceosyopince, I propose to take uj) the charactei-s of the 
skeleton as they have been described in the osteological part of this memoir. 
Dentition. — In Limnohyops laticeps the crowns of the molars are low. and then* 
is no constriction of the external V’s by a median buttress as in Tehnatotlicrium. \ 
This specimen is a portion of a mandible including teeth, which Dr. W. H. Scott infonnc<i me i- 
from the Wasatch. 
49 jonn. a. n. s. phila., vol. xi. 
