1 84 



EDENTA TA 



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, Nothrotherium escrivanense, 

 exceeding the largest existing Anteater, and the Megatherium 

 being larger than a Ehinoceros. 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 $ in Nothrotherium. 1 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 



FIG. 60. Section of upper molar teeth of Megatherium ameriwnum. x J. 

 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 grinding 

 surface the prominent ridge of hard dentine follows the external 

 contour, and is surrounded only by a thin layer of cement, as 

 in the existing Sloths. The Ground Sloths, as the members 



1 Lydekker, in Nicholson and Lydekker's Manual of Palaeontology, vol. ii. 

 p. 1299 (1889). Originally described under the preoccupied name Ccelodon. 



