wo | The Perissodactyla, 
A. No diastema 
Fourth ikesi premolar unlike the first true molar. 
oe inferior molar with five lobes; superior premolars 
RO A rises sic ccavengec curds cited «++... Systemodon Cope, 
AA. A diastema behind the first premolar in both jaws. 
. Last inferiot premolar different from first true molar. 
Fant inferior molar with heel; cross-crests of superior mo- 
lars interrupted ; Hy th 
ed 
aa. Last inferior premolar asad ae true molar. 
Dentition generally as in H Pliolophus Owen. 
II. External lobes of superior molars well separated and little flattened; inferior — 
molars with perfect cross-crests ( Protapirine). 
Last inferior molar with heel; ? four premolars ; Isectolophus 8, and 0. 
Last inferior molar without heel; inferior peak three ;..Protapirus Filb. 
III, FER lobes of superior molars flat, not well distingata forming a 
ferior molars with Aiea cross-crests (Lophiodontina). 
. i No diastema in lower j 
Last inferior molar with third lobe; Helaletes” Marsh. 
AA, Lower jaw with diastema. 
* No diastema behind first Sa es 
a. No inferior premolars like the true molars. 
Superior molars 
Last inferior molar with heel; Heptodon Cope. 
Last inferior molar without heel ; Hyrachyus Lay: 
rior molars 6, 
Last inferior molar with heel: Lophiodon. Cuv. 
Last lower molar without heels, no horns; ege _Dilophodon odon Scott. 
Last lower molar?; “an attachment for a Derat horn on 
each nasal bone;”... ,Colonoceras Marsh. 
The above table shows that the modification which this family 
-has undergone in its superior molars, has consisted in the cot 
ey fluence of the external tubercles into a more or less irreg 
: external wall to the crown, and the confluence of the bee 
ate and internal tubercles into cross-crests. In the lower i 
cross-crests have been formed. It is impossible to page 
Hyracotheriine sub-family as a family from the Lop as 
Since the characters grade into each other completely. ) 
has been from the Hyracotheriine sub-family that ee : 
Was derived; the Protapirine gave origin to the a 
while the AIMERA have descended from the Lopht Jh j 
Among Hyracotheriinæ the genus Systemodon (Cope a 
the lowest place on account of the entire absence al 
from the dental series, It is as old as any of ae genet’ 
