J 1887] The Perissodactyla. 1061 
_ I Internal cones of superior molars separate from external lobes. 
A, External tubercles subconic, separated by a vertical external tubercle. 
Fourth inferior premolar like first true molar;.........seseee Ectocium Cope. 
Third and fourth inferior premolars like the true molars; ... Æpihippus Marsh." 
AA. External tubercles of superior molars become V’s, which are separated ex- 
ternally by a vertical ridge. 
a. Incisors present. 
K 
$ 
: B. No diastema in front of second inferior premolar. 
Second premolar without inner lobe; last molar with one 
inner cone: : Leurocephalus S. & 0. 
Second premolar with inner cone; last superior molar with 
an inner cone; Palæosyops Leidy. 
Second premolar with inner cone; last superior molar with 
two inner cones; ; 
f imnohyus Leidy. 
88. A diastema in front of second inferior premolar. 
` . 4 O a 
Two inner cones of last superior molar ; bdoth Cope. 
aa. Incisors absent from both jaws. 
Last superior molar with one internal CONG Sartr E AAACN Nestoritherium Kaup. 
IL One or both internal cusps of superior molars united with the external lobes by 
cross-crests, : 
a. External cusps of superior molars more or less conic.. 
An antero-external cingular cusp; pamper ON 
aa, External lobes of superior molars, inflected V's. . 
= 8. No crescentic inner lobes. 
4 Intermediate lobes confluent ; 
Fic, 25 
WwW - Ectocium osbornianum Cope, molars, natural size; from the Suessonian 
" Wyomi ` 
ing. Fig. æ, superior molars; 4, inferior molars. Original. 
%6. Lambdotherium popoagicum Cope, lower jaw ramus, natural size; from 
River Eocene of Wyoming. Original. 
The p hylogeny of this family is not difficult to read. Ecto- 
: ‘cum, if it be truly a member of it (the feet are unknown), is 
tes) the primitive genus, which is not far removed from Sys- 
Saga of the Lophidontidz, in characters. The flattening of 
emal cusps produced the two external Y's of the other 
à and this, without further modification, would give us 
cephalus and Palzosyops, the former having the second 
'0r premolars more simple than in the latter. This type, 
t Teste Scott and Osborn. 
