148 
MESODONTA. 
In attempting the identification of this genus with one of those named 
by Professor Marsh, it appears that I have been in error. The absence of 
sufficient diagnosis in the single description yet published by this author 
explains my want of success. In several other instances, I have used the 
generic names of the “ Preliminary Description of New Tertiary Mammals, 
by Prof 0. C. Marsh”, bearing date August and September, 1872, hoping 
to be able to give them currency by supplying definitions. My want of 
success in these instances also, has more than ever convinced me of the 
importance of preserving the rule requiring definitions for genera as the 
only basis of authority for their names; and it is evident that to suspend 
this rule of nomenclature is impracticable, even were it desirable. I have 
therefore made a final examination of the names and descriptions of the 
above-mentioned paper, with the view of ascertaining which of them are 
introduced in such a way as to constitute additions to nomenclature. I find 
that of thirty-seven generic names proposed in that and a previous essay 
(On New Fossil Mammals and Birds from the Tertiary Formation, July and 
August, 1871), tAventy-two are not accompanied by a mention of their 
generic characters ; while of the remaining fifteen there are only two cases 
in which the nomenclator states what he regards as their distinctive features. 
These are Orotherium, which is adopted in the present work, and Telmato- 
lestes, the character of which, as given, is of A^ery doubtful value. To the 
list of undefined names must be added Tinoceras and Lemuravus, of later 
date. 
I may add that a feAv of the names early proposed by me, are open to 
the same criticism; but their publication Avas folloAved, at the earliest prac¬ 
ticable moment, by tlie issue of elaborate descriptions, in which the generic 
and specific characters are fully detailed. This has not yet (March 1, 1877) 
been done in the case of any of the names of the two papers aboA^e com¬ 
mented on. 
Tavo species are provisionally referred to the genus Sarcolemur on 
account of their resemblance to tlie knoAvn species S. jiyg^nceus, Cope, and 8. 
furcat'us^ Cope, but not definitely, because the premolar teeth are unknown 
in both cases. One of them is similar in size to the species mentioned, Avhile 
