434. Bulletin: American Museum of Natural History. [Vol. XXXIV, 
Notharctida TROUESSART 1879 1. c.; OsBorN 1902, Bull. A. M. N.H., Vol. XVI, 
p. 190; Srmx.tn, 1912, Abh. d. schw. pal. Ges., Vol. XX XVIII, p. 1287. 
Notharctus and its relatives in the American Eocene are considered as 
a distinct family by some authorities, as a subfamily of the Adapide by 
others. Doctor Stehlin in his recent monographic study of Adapis speci- 
fies a series of important distinctions in the character and evolutionary 
trend of the two groups and concludes that they are distinct. In addition 
to a series of less fundamental distinctions he points out that the hypocone 
in the upper molars originates in the Notharectid phylum by budding off 
from the protocone, whereas in the Adapide he believes that it is certainly 
a derivative from the cingulum. Doctor Gregory’s morphologic studies 
of the skulls and skeletons of Notharctus and its relatives indicate, however, 
a somewhat nearer affinity to Adapis, so that the two may be considered 
as divergent phyla of a single family. The reasons for this conclusion will 
be set forth by him in other articles in this Bulletin. They tend moreover 
to emphasize the lemuroid affinities of the family, upon which most authori- 
ties, with the exception of Dr. Wortman, are agreed. 
Three or more genera of Notharctidee accur in the Middle Eocene forma- 
tions. In the Lower Eocene formations there are two, Pelycodus character- 
izing the earlier horizons, Notharctus, typically from the Middle Eocene 
(Bridger) but represented by primitive species from the Wind River (Lost 
Cabin beds). The two genera are successive stages of a single phylum 
and are distinguished as follows: 
Notharctus Leidy: Hypocone and mesostyle of upper molars prominent. 
Pelycodus Cope: Hypocone and mesostyle rudimentary or absent. 
The species of Notharctidse from successive horizons of lower and middle 
Kocene, from the Sand Coulée to the upper Bridger illustrate very clearly 
the progressive change in the upper molars from tritubercular to fully 
quadritubercular type, the development of the mesostyle, the complication 
of the fourth premolar and increase in size. This was pointed out by 
Osborn in 1962,! and is confirmed and extended by the much larger series 
of specimens and exact records of horizons now at hand for comparison. | 
They are very good horizon markers. As with other abundant groups, a 
large series shows a certain range of individual variation, some being more 
and others less progressive, but within comparatively narrow limits. As 
we progress upward through successive levels of the formations, we find 
that the limits of individual variation, on one side and the other of the 
abundant typical forms, are progressively shifted over in the direction of 
the phyletie trend. That this is a gradual shifting of averages, due to the 
1 Osborn, 1902, Bull. Amer. Mus. Nat. Hist., Vol. XVI, p. 191. 
