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, ii., p. 43 (in 



part). 



This genus is easily distinguished, as Cope points out in his 

 sumptuously illustrated " Vertebrata of the Tertiary Forma- 

 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. SCOTTIANUS (Cope); all from the Eocene 

 of Wyoming. 



