122 COMPARATIVE DENTAL ANATOMY 



tection. They may have large numbers of teeth, 

 — ninety-six to one-hundred being found in some 

 species. These are of simple, molar-like form, in- 

 creasing in size from front to back, and are com- 

 pressed laterally, with oblique surfaces. Some- 

 times there are enamel germs in the embryo, but 

 these are absorbed as the tooth develops, so that 

 the tooth has no enamel. The teeth alternate, 

 and are worn in distinct facets. Some species, 

 perhaps all Armadillos, have a successive set. 

 The Sloths are hairy mammals of arboreal habits, 

 which suspend themselves beneath limbs by means 

 of the powerful claws, which are formed for hang- 

 ing down and moving about in trees. They have 

 few teeth, the formula being mostly m. j~ = 18, 

 implanted in the maxillary bones above and the 

 mandible below. They are of molar-like form, 

 and increase in size from front to back. The 

 tooth is composed of a central axis of vascular 

 dentin with an outside investment of harder 

 dentin, and grows continuously. The grinding 

 surface is usually cup-shaped, the hard rim pre- 

 senting two or more points. These curious mam- 

 mals are found in South America, and are prob- 

 ably descended from the great extinct Megathe- 

 roids of the Pampean formations. The gigantic 

 Megatherium (an immense Sloth of earlier geo- 

 logical times) had the same formula and the same 

 kind of teeth as the present sloths, but the 



