ed 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_.

Weld County, northeastern Colorado

[Illustration: FIG. 2. _Sinclairella dakotensis_ Jepsen, UCM no. 21073,
right M2; Orellan, Weld County, Colorado; drawing by Mrs. Judith Hood:
occlusal view, approximately x 9.]

_Material._--UCM no. 20173 (fig. 2), is a right M2.

_Locality and stratigraphy._--The tooth was discovered at
the Mellinger locality, Sec. 17, T 11 N, R 65 W, Weld
County, Colorado. The Mellinger locality is in the Cedar
Creek Member, White River Formation, and its fauna is
considered to be of Orellan age (Patterson and McGrew, 1937,
and Galbreath, 1953).

_Description and comments._--UCM no. 21073, which is more
heavily abraded than KU no. 11210, shows no evidence of a
stylar cusp either anterolabial to the metacone or in the
position of a mesostyle. A small stylar cusp is present
anterolabial to the paracone. A notch that appears to have
been cut through the enamel of the posterolabial corner of
the crown could have received the parastylar apex of M3. A
similar notch is not present on the M2 of KU no. 11210 nor
indicated in the illustrations of the M2 of Princeton no.
13585. The coronal dimensions of UCM no. 21073 (table 1) do
not appear to differ significantly from those of the M2's of
KU no. 11210 and the type specimen of _Sinclairella
dakotensis_.

Comments

With the discovery of Orellan apatemyids the geochronological range of
the family in North America is shown to extend from the Torrejonian
through the Orellan land-mammal ages. The discoveries reported here
enlarge the Oligocene record of apatemyids to include not only the type
specimen of _Sinclairella dakotensis_, a skull and associated mandible