122 ALLEN’S NATURALIST’S LIBRARY. 
This genus is characterised by the second pre-molar having 
always two roots; the anterior has one root and the third 
three ; the posterior has one external and one internal cusp. 
Of the true molars, all have two external cusps ; the anterior 
and median have two internal cusps and the posterior has only 
one ; of the lower teeth the posterior pre-molar has an internal 
cusp and a heel; the next one has no internal cusp; the 
molars often have the fore inner cusps double ; the posterior 
molar has a strong heel. This genus contains three species, 
all described by Cope (P. JARROvII, P. TUTUS, P. FRUGIVORUS), 
with the hind inner cusp of the upper molars distinct from the 
heel ; and P. ANGULATUS, in which that cusp is small and is 
on the heel. Their remains have been found in the Lower 
Eocene (Wasatch) beds of New Mexico. P. HELVETICUS has 
been described from the Upper Eocene of Egerkingen. 
GENUS MICROSYOPS. 
Microsyops, Leidy, Proc. Acad. Nat. Sci., Philad., 1872, p. 20. 
Limnotherium, Marsh, Amer. Journ. Sci., 1871, il., p. 43 (in 
part). 
This genus is easily distinguished, as Cope points out in his 
sumptuously illustrated “ Vertebrata of the Tertiary Iorma- 
tions of the West,” ... by the absence of the first (anterior) 
inferior pre-molar, and probably of the superior first pre-molar 
also. The canine tooth of the lower jaw is very large. ‘The 
posterior pre-molar has an internal cusp, and the molars two 
front inner cusps. There are three species, distinguished 
chiefly by size, M. sPIERIANUS (Cope), very small ; M. ELEGANS 
(Marsh), the largest, with seven teeth succeeding the canine in 
the lower jaw ; and M. scorrianus (Cope); all from the Eocene 
of Wyoming. 
