276 ANATOMY OF VERTEBRATES. 



anterior and posterior surfaces, from those of all the smaller 

 Megatherioids, in presenting two transverse ridges, fig. 216, d; 

 one of the sloping sides of each ridge being formed by the cement, 

 c, the other by the vascular dentine, v, whilst the unvascular den- 

 tine, d, as the hardest constituent, forms the summit of the ridge 

 like the plate of enamel between the dentine and cement in the 

 Elephant's grinder. The great length of the teeth, and concomi- 

 tant depth of the jaws, the close-set series of the teeth, and the 

 narrow palate, are also strong features of resemblance between 

 the Megatherium and Elephant in their dental and maxillary 

 organisation. In both these gigantic phyllophagous quadrupeds 

 provision has likewise been made for the maintenance of the grind- 

 ing machinery in working order throughout their prolonged exis- 

 tence : but the fertility of the creative resources is well displayed 

 by the different modes in which this provision has been effected : 

 in the Elephant, it is by the formation of new teeth to supply the 

 place of the old when worn out ; in the Megatherium, by the 

 constant repair of the teeth in use, to the base of which new 

 matter is added in proportion as the old is worn away from the 

 crown. Thus, the extinct Megatherioids had both the same struc- 

 ture and mode of growth and renovation of their teeth as are 

 manifested in the present day by the diminutive Sloths. 



c. Cetacea. Those Mammals which are properly called s Whales' 

 have no teeth, but horny substitutes in the form of plates, termi- 

 nating or fringed by bristles. Of these plates, called ( baleen ' and 



217 



Baleen-plates and Tongue of Piked Whale (Balanopterd) 



' whalebone,' fig. 217, h, the largest, which are of an inequilateral 

 triangular form, are arranged in a single longitudinal series on 

 each side of the upper jaw, situated pretty close to each other, 

 depending vertically from the maxillary bones, with their flat 

 surfaces looking backward and forward, and their unattached 

 margins outward and inward, the direction of their interspaces 



