184 EDENTATA 
dentition of the former with the structure of the vertebral column, 
limbs, and tail of the latter. Almost all the known species are of 
comparatively gigantic size, the smallest, Nothrotheriwm escrivanense, 
exceeding the largest existing Anteater, and the Megatherium 
being larger than a Rhinoceros. The femur has no third trochanter, 
and the odontoid process of the axis vertebra has a peculiar facet 
on the ventral surface. The dentition is usually = on each side, as 
in the Sloths, but 4 in Nothrotherium.! This genus, and in a still 
more marked degree Megatherium, differ from all the others in the 
details of the structure of the teeth. They are very deeply 
implanted, of prismatic form (quadrate in transverse section), and 
the component tissues—hard dentine (Fig. 60, d), softer vaso-dentine 
5 
EES 
PED iS SYA 
[Ltt Pee 
Fic. 60.—Section of upper molar teeth of Megatherium americanum. x}. 
p, pulp-cavity ; the other letters explained in the text. (After Owen.) 
(v), and cement (c)—are so arranged that, as the tooth wears, the 
surface always presents a pair of transverse ridges, thus producing 
a triturating apparatus comparable to the “bilophodont” molar of 
Dinotherium, Tapirus, Manatus, Macropus, and others, though pro- 
duced in a different manner. In all the other genera the teeth are 
more or less cylindrical, though sometimes laterally compressed or 
even longitudinally grooved on the sides, and on the erinding 
surface the prominent ridge of hard dentine follows the eden 
contour, and is surrounded only by a thin layer of cement, as 
in the existing Sloths. The Ground Sloths, as the mem heve 
1 Lydekker, in Nicholson and Lydekker’s Manual of Paleontology, vol. ii. 
p. 1299 (1889). Originally described under the preoccupied name Cwlodon. 
