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TEETH OF THE VERTEBRATA. 409

mark of degeneracy, and leads indirectly to the conclusion that the
armadillos at least are descended from ancestors with enamel-covered
teeth, who in all probability were the possessors of a completely devel-
oped second set.
The only assignable cause for this degenerate condition of the dental
organs is the peculiarity of their food-getting habits. Many of them
feed upon insects, which they capture by means of a long whip-like
tongue covered with the viscid secretion of the submaxillary glands, and
swallow whole. This manner of feeding would occasion little demand
for nuisticatory organs, which from disuse would gradually fall into a
rudimentary condition and eventually disappear. Those in which the
entomophugous habit is exhibited in its greatest perfection are edentu-
lous, and have small mouths with extremely long tongues. All the
Edentata in which this structure exists at all show a tendency toward
such a habit—even the arboreal sloths, which are said to be exclusively
vegetable feeders.
Flower has recently shown that the sloths are intimately connected
with the ant-eaters and armadillos of South America through the extinct
megatheroids, and that all the American forms have probably descended
from a common ancestor, while the Old World forms are likewise closely
related and descended in another line. It is prol^ably true that the arma-
dillos are most nearly related to the ancestral form, and that the sloths
represent an offshoot which was derived from them after they had lost
the enamel of the teeth in the manner indicated.
The teeth of the armadillos are, with one exception, relatively small
cylindrical bodies implanted in the dentigerous borders of the lower jaw,
m ixiljary, and sometimes premaxillary bones. They are entirely devoid
of enamel, and grow continuously throughout the life of the animal, in
consequence of which no roots are formed.
Fig. 198.

Side View of the Skull of a Seven-bamled Anuadillo {Tatusin hybrUhisf)
In the seven-banded armadillo (Ta/u-s/rt hybrid us, Fig. 198') there are
seven teeth above and eight belo^v upon each side. So little is known
' The specimen here figured is in the U. S. Army Medical Museum, and is labelled
Tatuxia st'plemcindiis. It exhibits tlie peculiarity of liaving eight teeth upon one side
and seven upon the other in the ui)per jaw. Tliere is, however, a con.siderable space
between the first and second tooth of the right side, wliich would indicate tiiat a tooth
is missing. Tlie number ascribed to this species by Owen is seven above and eight
bek)w upon each side. Its e.xact identification is therefore diflScult.