CETACEANS. 345 



hypothetical " vasa lymphatica" of the old physiologists, from the 

 ultimate blood-vessels, they form one continuous and freely inter- 

 communicating system of strengthening and reparative vessels, by 

 which the plasma of the blood was distributed throughout the entire 

 tooth for its nutrition and maintenance in a healthy state. 



The grinding surface of the close-set molars of the Mega- 

 therium differs, on account of the greater thickness of the cement 

 on their anterior and posterior surfaces, from those of all the 

 smaller Megatherioids, in presenting two transverse ridges ; one 

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

 other by the vascular dentine ; whilst the unvascular dentine, 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 concomitant 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 organization. In both these 

 gigantic phyllophagous quadrupeds provision has likewise been made 

 for the maintenance of the grinding machinery in an effective state ; 

 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 structure and mode of 

 growth and renovation of their teeth, as are manifested in the present 

 day by the diminutive Sloths. 



CHAPTER IV. 



TEETH OF CETACEANS. 



139. BalcEnida. — In this, as in the preceding order, the dental 

 system presents little fixity of character, and its variations extend 

 in some cases to anomalies. Surveyed in the true or carnivorous 

 Cetacea, it seems, on the whole, to be of a grade inferior to that 



