274 



ANATOMY OF VERTEBRATES. 



<>15 



proportion of the centre of the tooth, and being harder than 

 the dentine, d, or cement, c, rose upon the grinding surface, 

 in the form of a ridge extending along the middle of the long 

 axis of that surface, and in three shorter ridges at right angles to 

 the preceding, at the middle of each of the three rhomboidal 

 divisions of the tooth. 



Of the leaf-eating species of the order Bruta, very few, and 

 these the most diminutive of the tribe, now exist. The following 

 are the characters of their dentition, both recent and extinct : — 

 Teeth implanted in the maxillary and mandibular bones, few in 

 number, not exceeding £:•{ ; composed of a large central axis of 

 vaso-dentine, with a thin investment of hard dentine, and a thick 

 outer coating of cement : to these add the dental characters 

 common to the order Bruta, viz., uninterrupted growth, and con- 

 comitant implantation by a simple, deeply-excavated base. 



In the two-toed sloth {Cholcepus didactylus,I\\\g.) the teeth, 

 fig. 215, offer a greater inequality of size than has yet been 



observed in any other genus of Bruta ; 

 the first of each series, l, in both jaws, 

 which in the rest of the order is the 

 smallest, here so much exceeds the 

 others as, with its peculiar form, to 

 have received the name of a canine. 

 This tooth is separated by a marked 

 interval from the other teeth, 2-5, es- 

 pecially in the upper jaw, so that l-i 

 above play upon the anterior part of 

 those below, contrary to the relative 

 position and mutual action of the true 

 canine teeth in the Quadrumana and 

 Carnivora. 



The teeth of the Megatherium, the 

 most gigantic of the extinct quadru- 

 peds of the Sloth tribe, are five in number on each side of 

 the upper jaw, fig. 216, and four on each side of the lower jaw. 

 They are deeply implanted with narrow intervals : each is exca- 

 vated by an unusually extensive pulp-cavity, ib. p, from the apex 

 of which a fissure is continued to the middle depression of the 

 grinding surface of the tooth. The central axis of vaso-dentine, v, 

 is surrounded by a thin layer of hard or unvascular dentine, d, and 

 this is coated by the cement, c, which is of great thickness on the 

 anterior and posterior surfaces, but is thin where it covers the 

 outer and inner sides of the tooth. The vaso-dentine, v, fig. 238, 



Teeth of the two-toed Sloth {Chaloepus 

 didactylus). 



