_Material._--KU no. 11210 (fig. 1), a fragment of a left
maxillary containing P4 and M1-2.
_Locality and stratigraphy._--The fossil was found in the
center of the W-1/2, Sec. 21, T 11 N, R 53 W, Logan County,
Colorado, "... in the bed below _Agnotocastor_ bed, Cedar
Creek Member...." (Ronald H. Pine, 1958, field notes on file
at the University of Kansas). The bed so defined is part of
unit 3 in the lower division of the Cedar Creek Member, as
subdivided by Galbreath (1953:25) in stratigraphic section
XII. The fauna obtained from unit 3 is of Orellan age.
[Illustration: FIG. 1. _Sinclairella dakotensis_ Jepsen, KU no. 11210,
fragment of left maxillary with P4 and M1-2; Orellan, Logan County,
Colorado; drawings by Mrs. Judith Hood: a, labial view; b, occlusal
view; both approximately × 9.]
_Description and comments._--P4 of KU no. 11210 has a large
posterolingual cusp separated from the main cusp by a
distinct groove, which deepens posteriorly. The
posterolingual cusp is supported by the broad posterior
root. P4 of the type specimen of _Sinclairella dakotensis_
is described (Jepsen, 1934, p. 392) as having an oval
outline at the base of the crown, and a small,
posterolingual cusp. A chip of enamel is missing from the
posterior slope of the main cusp of the P4 of KU no. 11210.
The anterior slope of the main cusp is flattened, possibly
the result of wear, and there is no evidence of a groove
like that present on the P4 of the type specimen.
Only a few differences were found between the molars
preserved in KU no. 11210 and their counterparts in the type
specimen. A stylar shelf is present labial to the metacone
of M1 of KU no. 11210, but, unlike the type, its surface is
smooth and there is no evidence of cusps. Of the three small
stylar cusps on the stylar shelf of M2 the smallest is in
the position of a mesostyle. The M2 lacks a chip of enamel
from the lingual surface of the hypocone. Unlike the M2 of
Princeton no. 13585, in occlusal view the posterior margin
of the M2 of KU no. 11210 is convex posterior to the
metacone. The anterior edge of the base of the zygomatic
arch of KU no. 11210 was dorsal to M2. The shallow oval
depression in the maxillary dorsal to M1 might be the result
of post-mortem distortion.
The molars preserved in KU no. 11210 and their counterparts
in the type specimen do not appear to be significantly
different in size (table 1) or morphology of the cusps. The
only difference between the two specimens that might be of
classificatory significance is the difference in size of the
posterolingual cusp of P4. At present the range of
intraspecific variation in the morphology of P4 has not been
documented for any species of apatemyid. The evolutionary
trend or trends of the apatemyids (McKenna, 1960, p. 48) for
progressive reduction of function of p4 probably were
paralleled by similar trends in the evolution of the P4. If
so, the intraspecific variation in the morphology of P4
could be expected to be somewhat greater than that of the
upper molars, for example. The morphological difference
between the P4's of the type of _Sinclairella dakotensis_
and KU no. 11210 is not extreme and does not exceed the
range of intraspecific variation that could be expected for
this element of the dentition. The close resemblances in
size and morphology between the M1-2 of Princeton no. 13585
and KU no. 11210 also favor identification of the latter as
part of a member of an Orellan population of _Sinclairella
dakotensis_.