site humours after the central pulp has almost ceased to ex ist.” In the Megatherium,
liovvever, those anastomoses have not to perform a vicarious office, sin ce the
pulp maintains its full s iz e and functional activity during the whole period o f the
animal’s existenc e. It relates to the higher organized condition, and greater degree
o f vitality o f the entire grinder in that extinct species.
Th e conical cavities (d. PI. X X X I .) a ttest the size and form o f the persistent
pulp ; the diameter o f its base is equal to the part o f the crown o f the tootli
which is formed b y the coarse and fine dentine. From the gradual thinning oft',
and final disappearance o f these substances as they reach the base o f tlie tootli,
I conclude that they were both formed at the expense o f the pulp. Tiie fine tubes
and cells must have been excavated in its peripheral layer for the reception o f the
liardening salts o f the dense dentine, and the rest converted into the parallel series
o f medullary canals with their respective systems o f calcigerous tubes, in a manner
closely analogous to the development o f the entire tooth o f the Orycteropus.
The coarser dentine o f the tooth o f the Megatherium ditfers, in fact, from the
entire tooth o f the Orycteropus, only in that the parallel medullary canals and their
radiating calcigerous tubes are not separated from the contiguous canals by a
distinct layer o f cæmentum, and that the medullary canals anastomose at their
peripheral extremities. T h e wide spaces, (e. PI. X X X I .) indicate the thickness
o f the dental capsule by the ossification o f which the exterior stratum o f cement was
formed. It was not until I knew the true structure o f the tooth o f the Megatherium,
that I could comprehend the mode o f its formation. The parallel layers of
enamel in the Elephant’s grinder are formed, as is well known, by membranous
plates p assing from the coronal end o f the closed capsule towards the base o f the
tooth ; but a certain extent o f enamel can only thus be formed, and when the
crown o f the grinder has once protruded, and come into use, the enamel cannot
be added to. The modification o f the structure o f the tooth o f the Megatherium
readily permits the uninterrupted and continuous formation o f the dense sub stance
whicli is analogous to the enamel o f the Elephant's grinder.
With respect to the question o f the respective affinities o f the Megatherium
to the Bradypodoid or Dasypodoid families, the result o f this examination o f the
teeth speaks strongly for its closer relationship with the former group: the
Megalonyx, Mylodon, and Scelidotherium, in like manner correspond in the
structure o f their teeth with the Sloth, and differ from the Armadillo.
I f from a similarity o f dental structure \ve may predicate a similarity o f food,
it may reasonably be conjectured that the leaves and soft succulent sprouts o f
trees may have been the staple diet of the Megatherioid quadrupeds, as o f the
ex isting Sloths. Their enormous claws, I conclude, from the fossorial character
of the powerful mechanism by which they \vere worked, to have been employed,
'.Ih'