Marsh Collection^ Peabody Museum. 353 



Leidy, at the time he proposed the genus Mierosyops^ thought 

 to be identical with the specimens he had in hand, and adopted 

 Marsh's specific name gracilis. Marsh, however, in the same 

 paper in which he described Ilyopsodus gracilis, had proposed 

 another species, Limnotherimn elegans. From an examina- 

 tion of Marsh's types, Leidy afterward concluded that it was 

 to L. elegans that his specimens were to be referred, and that 

 Hyojysodics gracilis was a different species. His exact words 

 are:* "The specific name of M. gracilis was originally given 

 under the impression that the remains referred by Professor 

 Marsh to Ilyopsodus gracilis pertained to the same [species 

 of] animal. A specimen exhibited to the writer by Professor 

 Marsh would indicate that M. gracilis is the same as the 

 animal named by him Limnotheriicm elegans. As Microsyops 

 is generically distinct from Limnotherimn as characterized 

 from the typical species, L. tyrannus, the specific name of the 

 former would be Microsyops elegansT 



A careful examination of the types confirms Leidy 's con- 

 clusions as given above, and establishes the further important 

 fact that Ilyopsodus gracilis of Marsh is not only distinct 

 speciiicall}^ but represents an apparently undescribed genus of 

 the Microsyopsidse. The oldest members of this group come 

 from the second stage of the Lower Eocene, or Torrejon beds, 

 of New Mexico. The first species of this group found was 

 described by Cope as Mixodectes.\ Quite recently Osborn 

 has added a second genus Olhodotes.X The chief characters 

 of Mixodectes, which is known almost exclusively from lower 

 jaws, are tlie following: There are eight teeth in the jaw, of 

 which three are molars, three are premolars, one is a canine, 

 and one an incisor; the last premolar is much simpler than the 

 molars in structure ; the two incisors, representing the central 

 pair according to Osborn, are m'oderately enlarged. 



Olhodotes has a full incisor dentition in the lower jaw, with 

 a tendency to enlargement of the central pair. The premolars 

 are reduced to two and the fourth premolar is simpler than in 

 Mixodectes. It is therefore the most primitive species of this 

 group thus far known, if correctly referred to this series. 



From the succeeding Wasatch, Cope has described another 

 genus under the name of Cynodontomys. This species, while 

 very much like the Torrejon Mixodectes, differs from it in 

 having lost either the canine or the second premolar and in the 

 greater enlargement of the incisors. 



In the Wind River, we have the first appearance of the 

 genus Microsyops, which differs from Cynodontomys in the 



* Extinct Vertebrate Fauna of the West, 1873, p. 84. 

 fAmer. Philos. Soc, 1882-1883, p. 550. 



i American Eocene Primates, etc., Bull. Amer. Mus. Nat. Hist., 1902, p. 

 206. 



Am. Jour. Sci. — Fourth Series, Vol. XVI, Xo. 95. — November, 1903. 

 25 



