SLOTHS. 331 



do not present the curvatures observable in the lateral layer, but 

 pass almost in a straight line to the grinding surface of the tooth, 

 slightly converging and decussating each other at the middle, and 

 slightly diverging near the margin of the crown. The coronal portion 

 of the hard dentine, which is originally nearly three times the thickness 

 of the cylindrical or parietal portion, is worn away almost before 

 the Sloth reaches maturity ; and, as the pulp, when once it has 

 taken on the incomplete process of calcification which produces 

 the vascular cement, is never afterwards subject to the primitive 

 and more complete process, the coronal hard dentine is never re- 

 produced. The depressed centre of the grinding surface is then 

 formed by the vascular dentine only; that surface having its inequality 

 maintained by the edge of the cylinder of hard dentine, in place of 

 enamel, and being thus adapted to the comminution of the softer 

 vegetable substances, as the leaves and tender buds of trees. (1) 



The external coat of cement has an average thickness of Jth of 

 a line ; it gradually thins off to the sharp margin of the pulp-cavity, 

 forming the inserted base of the tooth. It is principally charac- 

 terized by the numerous calcigerous cells, represented by the dark 

 or opake dots in Plate 82, fig. 1. These present a more or less 

 oblong form, with the long axis parallel with that of the tooth: 

 those next the hard dentine making the nearest approach to the 

 circular form. Their average diameter is 3^0^^^ of an inch in the 

 long-diameter, and ^^th in the short diameter; but the extremely 

 numerous minute tortuous tubes, which open into every part of 

 their circumference, give the cells a greater apparent size than they 

 present when viewed by a sufficiently deep power for the clear 

 analysis of the rich and minute tubular system, with which they 

 are everywhere connected. The principal trunks of this system, 

 which, nevertheless, do not exceed the diameter of the calcigerous 

 tubes of the hard dentine, affect a parallel course in many parts 

 of the cement, at nearly right angles to its surface. They are most 

 conspicuous at that part of the cement, which is in contact with 



(1) Cecropia peltata and Achra sapoia have been particularized as affording nutriment 

 to tlie Sloths ; but, probably, few of the trees forming the denser forests of tropical America 

 are exempt from their attacks. 



