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430 DENTAL ANA TOM Y.

(IlacrosceHdce) the molars are quadritubercular both above and below,
and exhibit no traces wliatever of the complex W-structure. If it is
imperative to make any division of the Insectivora upon the characters
of the teeth, I would suggest that the W-arrangement of the cusps
of the superior true molars be considered as available for the purpose,
although I would be seriously disposed to question this character alone
as indicative of community of descent.
Another family of the Insectivora which in all probability stands in
ancestral relationship to the Carnivora is the one which Cope calls the
Miacidce. It is represented by two genera, Dklymiciis and Miaci.s, both
from the Eocene of North America. In this family we have, as Cope
remarks,^ "the point of nearest approximation of the Creodonta and
Carnivora. This is indicated by the fact that
the sectorials are sectorials both by position
and form, such as are not elsewhere met ^^ith
in the Creodonta. The genera might readily
be taken for members of the Canidfe and Vi-
verridte (dogs and civets) but for the struc-
ture of the astragalus, which is thoroughly
creodont."
The genus Didymictm may be certainly re-
garded as the ancestor of the civets, while it is
Fragment of the Lower .Inv.- of a
more than probable that Miach (Fig. 213) was
species of Jfincis : a, external, /<,
internal, and c, vertical views.
the immediate progenitor of the dogs. In the
teeth of Didymidis (Fig. 214) the dental formula is not comj^letely
Fig. 214.

ft, h, c, internal, vertical, and external views ol lower jaw of D. dawhin-
Two Species of Didt/micfis :
sianiis, from I'ig Horn Beds ; il, e, /, I), /ini/cleniami.s ; d, upper-jaw fragment, vertical view ; e, frag-
ment of lett ramus, inner side; /, vertical view of same,—all natural size (after Cope).
known, but mo.st probably it is I. f , C. \, Pm. f , M. f = 40. The
fourth premolar above is sectorial in form, the two true molars tuber-
cular.
In tlie lower jaw the tirst true molar has lost much of its typical
tuberculo-sectorial structure, which is best seen in the decreased size of
the internal tubercle and the tendency of the anterior basal lobe and the
^ "The Creodonta," American Naturaliftt, May, 1884, p. 483.