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412 DENTAL ANATOMY.

exception that the three posterior ones are more robust and gradually
decrease in size from the second to the last. Viewing the teeth and
accessory bony structures of this animal as a whole, the premaxilke are
remarkable for their small size, little extension anteriorly beyond the
maxilla?, and the complete absence of the ascending or nasal process, as
well as their edentulous condition. The palatal plates of the maxillae
are Ayidest in front and gradually narrow posteriorly, causing the dental
series of opposite sides to conyerge behind. In the lower jaw the two
halyes are completely co-ossified, as in monkeys and man ; the anterior
part of the symphysis is produced into a peculiar spout-like termination,
at the base of which the jaw widens rapidly ; and the rami are little
diyaricated posteriorly. The posterior teeth are implanted in a strong
inwardly projecting "ledge, in consequence of which the dentigerous
border gradually approaches the median line as it proceeds bac.k\\ard.
The mental foramen is placed below the interyal between the third and
fourth teeth near the middle of the ramus.
With respect to thg teeth themselyes, Owen giyes the following com-
mon and constant characters of both recent and extinct sloth-like ani-
mals, which would include the megatheroids : " Teeth implanted in the
maxillary, neyer in the intermaxillary bones ; few in number, not ex-
ceeding f zf ; composed of a large central axis of vascular dentine,
with a thin inyestment of hard or unvascular dentine, and a thick outer
coating of cement. To these, of course, may be added the dental cha-
racters connnon to the order BruUi—viz. uninterrupted growth of the
teeth, and their concomitant implantation by a simple deeply-seated exca-
vatecl base, not separated by a cervix from the exposed summit or crown."
Of the two Old Workl genera now living, but one has teeth. This
is the aard-vark (Orycterojms), or, as it is sometimes called, the Cape
ant-eater. Its dental formula is M. |^z|=^26, of which the anterior
ones of each series are not unfrequently wanting or concealed by the
gum. The teeth of the superior set progressively increase in size from
before backward up to the last tooth, which is smaller. They are oval
in section, M'ith the exception of the fourth and fifth, and have Avedge-
shaped triturating surfaces, like the armadillos. The fourth and fifth
above and the last two below have two vertical grooves, one upon each
side, which give to them an hour-glass shape upon section.
The teeth of this animal do not exhibit the customary excavated base
of the Edentata generally, but are continued solid to the bottom of the
sockets. Their minute structure is peculiar, and resembles that found in
3Ii/Uohatrs among the elasmobranch fishes ; the dentine is of the variety
known as plici-dentine. This consists of a series of small vertical par-
allel tubnli which ])ass up from, and are virtually prolongations of the
pulp-cavitv. From these the dentinal tnbuli radiate toward the per-
iphery, just as they do from the single ])ulp-cavity <»f the human tooth
already described. Owing to this peculiarity, Prof. Owen regarded the
tooth of Orycferopu^ as an aggregate of many denticles, each with its
proper pulp-cavity and dentinal tubes.
In Eurojie fossil remains of edentates are known from the ISIiddle
Miocene of Sansan in France and the upper INIiocene deposits of Piker-
mi in Greece. Two genera, 3Iacrotherium and Ancylotherium, have been