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_On the Affinities of Leptarctus primus of Leidy._

By J. L. WORTMAN.



_AUTHOR'S EDITION, extracted from_

BULLETIN OF THE AMERICAN MUSEUM OF NATURAL HISTORY,

VOL. VI, ARTICLE VIII, pp. 229-231.


_New York, July 30, 1894._




Article VIII.--ON THE AFFINITIES OF LEPTARCTUS PRIMUS OF LEIDY.

By J. L. Wortman.


Up to the present time but very little has been known of the existence
of the peculiarly American family Procyonidæ in any deposits older
than the very latest Quaternary. Leidy has described and figured[1] an
isolated last upper tooth, from the Loup Fork deposits of Nebraska,
under the name of _Leptarctus primus_, which has been referred to this
family. The Museum Expedition of last year into this region was
successful in obtaining additional material, which we provisionally
refer to Leidy's species.


=Leptarctus primus= _Leidy_.

The specimen consists of the right ramus of the lower jaw, carrying
the third and fourth premolars and the canine. The condyle is broken
away, but the coronoid process and the angle are preserved. The
specimen is from a young individual in which the last premolar had
just cut the gum. The alveoli of all the other teeth are present and
in a good state of preservation.

The dental formula is as follows: I._3, C._1, Pm._3, M._2. The
incisors are not preserved, but their alveoli indicate that they were
much crowded, the outside one being placed almost directly in front of
the canine, and the middle one pushed back considerably out of
position. This series is in marked contrast with that of the Raccoon,
in which the crowns of the incisors form almost a straight line across
the jaw, and the middle one is crowded backwards to a very slight
extent. The canine is peculiar and differs markedly from that of the
Raccoon. It is rather robust, very much recurved and grooved by a deep
vertical sulcus upon its antero-internal face. This sulcus is but
faintly indicated in the Raccoon. The postero-external face of the
crown is marked by a sharp ridge which becomes more prominent near the
apex. The first premolar is not preserved, but its alveolus indicates
that it was a single-rooted tooth, placed behind the canine after the
intervention of a very short diastema. The second premolar is
bifanged; its crown is composed of a principal cusp, to which is added
behind a small though very distinct second cusp. There is in addition
to these cusps a distinct basal cingulum, most prominent in the region
of the heel. The third premolar, like the second, is double rooted;
its crown moreover is made up of two cusps, the posterior being almost
as large as the principal one. These cusps do not stand in the line of
the long axis of the jaw, but are placed very obliquely to it. The
heel is not very prominent, but the basal cingulum is well developed,
both in front and behind. As compared with the Raccoon, the second
premolar is more complex in that it has two cusps instead of one. In
the third premolar the posterior cusp is much better developed, and
placed more obliquely than in the corresponding tooth of _Procyon_;
the heel is moreover not so broad.

The first molar is not preserved, but judging from the size of its
roots it was decidedly the longest tooth of the series. The second
molar was likewise bifanged but much smaller; it was placed close
against the base of the coronoid.

The whole jaw has, relatively, a greater depth than that of the
Raccoon, and is remarkably straight upon its lower border, whereas in
the recent genus it is considerably curved. The condyle is not
preserved, and the angle is somewhat damaged, but it was apparently
not so strongly inflected as in the Raccoon. The masseteric fossa is
deep and prominent, and the coronoid is high and broad. The inferior
dental canal is placed higher than it is in the Raccoon, being
slightly above the tooth line. The symphysis is relatively deeper and
more robust than in _Procyon_, and the chin is heavier and more
abruptly rounded.

The jaw of _Leptarctus_ differs from that of _Cercoleptes_ in the
following characters: the coronoid is broader and of less vertical
extent; the condyle is not placed so high; the angle is elevated above
the lower border of the ramus, which is straight and not concave as it
is in _Cercoleptes_. In the depth of the symphysis and abrupt rounding
of the chin the two genera are similar. _Cercoleptes_, moreover, has a
moderately deep groove upon the antero-internal face of the canine,
but differs from that of _Leptarctus_ in having an external groove as
well. _Cercoleptes_ again resembles _Leptarctus_ in having only three
premolars in the lower jaw; the middle one, however, has only a single
cusp upon the crown, whereas _Leptarctus_ has two.

As compared with _Bassaricyon_,[2] the jaw is more robust, shorter and
deeper, with a more prominent chin. The two genera differ again in the
number of premolars.

Altogether, _Leptarctus_ appears to offer a number of transitional
characters between the more typical Procyonidæ and the aberrant
_Cercoleptes_. This is especially to be seen in the proportions of the
jaw, the reduction of the number of premolars, the reduction in size
of the last molar, as well as the depth of the mandibular symphysis.


FOOTNOTES:

[1] Extinct Fauna of Dakota.

[2] See J. A. Allen's paper, Proc. Phil. Acad., 1876, p. 21.


  Transcriber's Note:

  "Quartenary" was amended to "Quaternary" in the first paragraph.