PLATE XXIV. 



Reduced views of highly magnified sections of the teeth, showing their charac- 

 teristic structure in existing and extinct Phyllophagous Bruta. 



Fig. I. A portion of a longitudinal section, including rather more than 

 one-half of the breadth of the tooth of the three-toed Sloth {Bradypus 

 tridactylus) , seen with a magnifying power of 250 linear dimensions. 



a'. The apex of the persistent pulp-cavity. 



a. The central constituent of the tooth called vascular dentine. The 

 dark oblong spots represent obUque sections of the vascular or 

 medullary canals, which are between g^th and g^th of an inch 

 in diameter, with intervals of twice or thrice that breadth : these 

 "•anals proceed in a slightly undulating and subparallel course from 

 the internal to the external surface of the vascular dentine, and 

 contain processes of the pulp. Those which proceed from the 

 summit of the pulp are parallel or nearly so with the long axis of 

 the pulp, whilst those from the base of the pulp are transverse to 

 that axis : the intermediate canals have an intermediate course. 

 They everywhere send oif minute calcigerous tubes, and terminate 

 for the most part by splitting up into a pencil of smaller wavy 

 Branches, close to the hard dentine, the calcigerous tubes of which 

 are formed by the ultimate subdivision of those branches. 



6. The hard or unvascular dentine : it has the same general struc- 

 ture as the ordinary dentine or tooth-bone of the human teeth, 

 consisting of a clear substance permeated by tine calcigerous 

 tubes. 



c. The caementum. The thickness of this layer does not quite equal 

 that of the, hard dentine. It is principally characterized by the 

 numerous minute calcigerous cells which are represented by the 

 dots in the figure. These cells present a more or less oblong form, 

 with the long axis parallel with that of the tooth : their average 

 diameter is 3-^^th of an inch in the long diameter by x^ooth of 



