r 1887] The Perissodactyla. 1063 
E 
The known genera of M # are all American. They 
_ differ as follows : 
= @. Last superior premolar only with two inner tubercles. 
I Incisors present; no horns on the muzzle3........cccsccececeeeseenereee Diplacodon Marsh. 
: aa, All the superior premolars with two interior cusps. 
_ Six inferior incisors; canines very large; Deodon Cope. 
_ Six inferior incisors ; canines very small; horns on the muzzle;...Menodus Pomel. 
 Noinferior, and four small superior incisors; canine very small; 
4 orns on the muzzle £ Symborodon Cope. 
Diplacodon, in its simpler premolars, approaches the Chalico- 
_ theriidæ, and is the oldest of the American genera. It is from the 
f Diplacodon bed or Upper Eocene. Menodus and Symborodon, 
Which include some species of gigantic size, belong in the White 
 Riveror Oligocene, while Dæodon has, so far, only been obtained 
_ from the John Day or Middle Miocene. The phylogeny of the 
family is simple, as Diplacodon is clearly the ancestor of Daodon 
_ onthe one hand and Menodus on the other. Menodus in time, 
_ bythe loss of its incisors, gave origin to Symborodon. This line 
4 left no representatives later than Miocene time. If Epihippus 
4 enters this family, it may be the parent of Mesohippus of the 
next higher horizon, the White River Miocene (Oligocene). 
’ ere are numerous species of the genera Menodus and Sym- 
_ borodon, and they are among the most remarkable of Mammalia. 
They are readily distinguished, among other characters, by the 
‘orm of the horns. In one group of species they are round ex- 
pt at the tips, and are greatly elongated; in another they are 
sub-round or slightly compressed; in a third type they are short 
and trihedral; in a fourth type they are much compressed and 
expanded transversely ; and in a fifth they are of insignificant size. 
Four of these types exist in both genera. They may be compared 
-8s follows: 
Group rt. i Group a ; 
Menodus........4/. dolichoceras S. and O. M. coloradoensis Leidy. 
M. giganteus Leidy. 
M. tichoceras S. and O. 
M. angustigenis Cope. 
Symborodon....S. acer Cope. S. altirostris Cope. 
S. bucco Cope. 
Menodus........ a Group 3. L nue - oe Group 5- 
Jy v... M. ingens Marsh. . platyceras S. . 
Symborodon...s. trigonoceras Cope. S, Aeloceras Cope. 
VOL. XXI.—No, 12, 72 
