SLOTHS. 329 



dentine, occupying rather more than half the thickness of the 

 tooth, which is enclosed by a wall of unvascular dentine, and this, 

 by one of cement, of less thickness than the layer of hard dentine : 

 but this description applies only to the teeth of the mature animals ; 

 and the large anterior laniiform molars of the Unau have a very 

 thin coating of dark coloured cement. Before the teeth are abraded 

 by use, they all present a conical form, and consist chiefly of the 

 hard dentine, which is covered by cement, and has its cavity lined 

 by a layer of the unvascular dentine. Their resemblance at this 

 stage, to the ordinary cuspidate teeth, only wants the interposed 

 layer of enamel, between the coronal cement and the hard dentine 

 to be complete. The cement is soon abraded from the protruded 

 apex, and the exposed hard dentine is worn into a hollow ; the 

 magnified section of the tooth of the young Bradypus figured in 

 Plate 82, shows its structure at this stage of growth. Whilst 

 abrasion of the summit proceeds, the base elongates by progressive 

 addition of the three constituent substances ; and, when it has 

 attained the diameter of the adult tooth, these continue to be 

 added without variation of their respective thicknesses ; the complex 

 matrix alone preserving its conical form, and filling the cavity at 

 the base of the tooth. When the coronal hard dentine is worn 

 away, this constituent is afterwards added only to the vertical 

 surface of the central axis of vascular dentine, which henceforward 

 constitutes the central and deepest part of the grinding surface of 

 the tooth. 



The intimate structure of the vascular dentine resembles that 

 of the entire tooth in Psammodus, Ptychodus, and some other 

 Cartilaginous fishes, and that of the inner half of the dentine in 

 the Iguanodon : it is perforated by vascular or medullary canals, 

 from eloth to ^^oth of an inch in diameter, with intervals of twice 

 or thrice that breadth ; which canals proceed in a slightly undu- 

 lating and sub-parallel course from the internal surface connected 

 with the pulp, to the hard dentine, at right angles, for the most 

 part, with the plane of the pulp-surface, and obliquely to that of 

 the hard dentine : those vascular canals proceeding from the summit 

 of the pulp are parallel, or nearly so, with the axis of the tooth, 



