: 316 Bulletin American Museum of Natural History. [Vol. XXXIV, _ 
of H. brownt. The others are retained as species or subspecies and two 
new forms are added. | 
Osborn in 1902 pointed out the evolutionary progress observable in 
the species of Hyopsodus from successive stages of the Lower and Middle 
Eocene; this is in general confirmed and extended by the far larger collec- 
tions now available and the somewhat wider geologic range of the genus; 
but it is evident that not one but three or four phyla are present in each 
horizon; the relations of the Lower Eocene species to those of the Middle 
Eocene are not wholly clear, and the geological overlap of stages of each 
structural phylum suggests rather progressive displacement of older by 
newer stages coming in from some other region, than gradual evolution in 
loco. It might equally well be interpreted as the displacement of older 
by newer “mutants”, in the DeVriesian sense of this term. 
‘However this may be, the Lower Eocene species are distinguished from 
those of the Middle Eocene by the less molariform premolars, and this is 
most noticeable in H. s:mplex from the lowest horizon, while the Lost Cabin 
species approach nearest to those of the Bridger. In H. simplex the hypo- 
eones of the upper molars are smaller, the lower molar heels are more 
distinctly basined, m® is small and the entoconid of m3 is not distinct from 
the hypoconulid, characters lost in the later species and indicating affinities 
with the Paleocene Miocleznide, and with the more or less intermediate 
genus Haplomylus. 
Ameghino! has referred to the Hyopsodontidee the genus Selenoconus ’ 
of the Notostylops horizon in Patagonia, considering it as a separable with 
difficulty from Hyopsodus. His figures and descriptions indicate, however, 
that the lower molars in this genus had the characteristic and peculiar con- 
struction of the Notoungulata, which I will have occasion to discuss in a 
later section of this revision. Schlosser? refers Selenoconus to the Archze- 
opithecide, and figures the upper and lower teeth of Oldfieldthomasia, a 
closely related if not identical genus. Such resemblances as appear between 
this genus and the Hyopsodontidse may perhaps indicate common descent 
from the Miocleenidee but probably not any closer relationship. 
Key to Species of Hyopsodus. 
A. Hypocone small on m', absent on mi, 
a. M?® and heel of m3 very small, no entoconid on ms. 
Tien NG ye PO ee eae Oa RS hat PCR Ean eo. H. simplex. 
B. Hypocone well developed on m2, small on mi, : 
1 Ameghino, 1906, Anal. Mus. Nac. Buenos Aires, t. XV, (3° sér, t. VI) p. 291, figs. 72-3. 
* Schlosser, 1911, in Zittel’s Grundztige d. Pal., Vertebrata, p. 517. 
