sheath of hard unvascular dentine, about one line and a half in thickness,
with an exterior covering of cement, about half a line in thickness.
The vascular dentine is permeated, as in the Sloth, by long,
nearly straight and parallel vascular canals, proceeding, for the most
part outwards, and with a slight inclination, to the grinding surface ■
this inclination is least at the base, and greatest at the summit of the
tooth, where the vascular canals are parallel with the axis of the tooth.
They have a slightly undulating course ; a diameter of ^th of an
inch, which they preserve throughout with little variation; and
have intervals of about twice that diameter. They differ chiefly
from the vascular canals in the Sloths, by anastomosing together
in regular loops, turned towards, and close to the inner surface
of the hard dentine. Very fine calcigerous tubes are given off
into the interspaces of the vascular canals, from every part of their
course; but the most abundant penicilli of these tubes proceed
from the convexity of the loops, and there diverge to enter the
hard dentine, through which they then proceed in a parallel
course, forming the calcigerous tubes of that substance. These
tubes are almost as minute as in the Sloth, having a diameter of s4th
of an inch, but have a straighter course ; throughout the greater
proportion of the hard dentine, they run at right angles to the plane
of the cement. In the teeth of the Mylodon Darwinii, I observed in
the hard dentine a few small vascular canals, which had an irregular
course, occasionally anastomosing together, and branching at acute or
right angles; their diameter equalled that of five or six of the calcigerous
tubes: they were more numerous near the central part of the hard
dentine, were filled with dark carbonaceous matter, and, being continued
from the larger vascular canals of the soft dentine, established a communication
between them and the vascular canals of the cement.
In the Sloth, no vascular canals have been detected in the hard
dentine or in the cement; in the Mylodon a few of these canals,
of larger diameter than those of the hard dentine, are present in
the cement (1). They are directed towards the plane of the dentine,
and are most conspicuous near that substance; but do not form
a series of loops, like the vascular canals of the central substance.
(]) Memoir on the Mylodon, pi. xxiv. fig. 3, d.
and are fewer in number. The radiated calcigerous cells are as
numerous as in the cement of the Sloth’s molar, but offer more
conspicuously the elongated form ; their long axis being parallel,
as in the Sloth, with that of the tooth itself. They measure ^ th
of an inch in the long diameter. The cement, in the Mylodon, is
traversed likewise by numerous and close-set calcigerous tubes,
continued from those of the dentine, with a general direction transversely
to the surface of the cement; hut with a more wavy and
less parallel course, with more frequent bifurcations and more numerous
branches, the sub-divisions of which form a rich plexus
around each calcigerous cell. The dentinal cells are unusually conspicuous,
presenting the appearance of a network in a thin section of
the hard dentine of these fossil teeth: they present a sub-hexagonal
form, and a diameter of j^th of an inch ; and are figured with the few
dispersed vascular canals of the hard dentine in Plate 79.
Scelidotherium.—The genus Scelidotherium includes three known
species, which resemble the Mylodon more nearly than the Megalonyx,
but differ from both those genera, not only in the form of the teeth
but in that of the astragalus, and in the structure of the knee-
joint. The dental formula is the same as in the Sloth and Mylodon,
viz. ^ .(l) The sockets of the upper jaw are much closer together
than in the Mylodon, and the first is not separated by a wider
interval from the rest: they occupy an antero-posterior extent of
three inches, seven lines, in the Scelidotherium leptocephalum, and,
in the Mylodon robustus, one of five inches, four lines : yet the first
and second molars in the Scelidothere exceed those in the Mylodon
in size ; hut the rest are of inferior size in the Scelidothere,
and the last is the least of those in the upper jaw, contrary to its
proportions in the Mylodon. The four teeth of the lower jaw are
also in closer order than in the Mylodon ; the length of the alveolar
series is three inches, ten lines in the Seel, leptocephalum, and five
inches in the Myl. robustus, but the longest transverse diameter
of the first tooth in the Scelidothere exceeds that in the Mylodon,
whilst that of the last tooth is half an inch shorter than in the
Cl) Plate 80, figs. 1 and 2. The name Scelidotherium, has been left out, by mistake, at the
foot of this plate, in reference to the above figures.
z