SLOTHS. 327 



or unvascular dentine, and a thick outer coating of cement. To 

 tliese, of course, may be added the dental characters common to the 

 order Bruta : viz. uninterrupted growth of the teeth, and their 

 concomitant implantation by a simple, deeply excavated base, not 

 separated by a cervix from the exposed summit or crown. 



In the mature Ai, {Bradypus tridactylus, Linn.) the teeth(l) are 

 small, of a simple columnar form, presenting, for the most part, 

 a sub-elliptical transverse section, sub-equal, and separated from 

 each other by short intervals. The grinding surface has a central 

 depression, with a raised margin, w^orn unequally into one or two 

 points. The first tooth in the upper jaw is the smallest, the second 

 is the largest ; both approach the trihedral form. The first in the 

 lower jaw is more curved than the rest, and is compressed from 

 before backwards, the posterior surface forming a slight angle, and 

 bevelled down obliquely from the anterior margin, which is trenchant, 

 and sometimes notched : the fourth or last molar of the lower jaw 

 is subtetragonal, and rather larger than the rest. The teeth of 

 the upper and lower jaw are not opposite ; but subalternate when 

 the mouth is closed. 



The dental formula of the genus Bradypus is 



Inc. I, Canini I, Molares 3 = 18. 



Dr. Brant(2) has described and figured the skull of a young 

 Ai, in which a very small tooth preceded the compressed one on 

 each side of the lower jaw, rendering the number of teeth equal 

 to that in the upper jaw. The remains of the alveoli of these teeth 

 are visible in the jaw of a young Ai in the Hunterian Museum, but 

 they are soon shed ; and, if constant in the species, are confined 

 to the immature period. 



In the two-toed Sloth or Unau, {Cholapus didactylus, lUig ) 

 the teeth (3) offer a greater inequality of size than has yet been 

 observed in any other genus of Bruta ; the first of each series, in 

 both jaws, which in the rest of the Order is the smallest, here so 



(1) PI. 81, fig. 1. 



(2) Dissertatio Zoologica inauguralis de Tardigradis, 4to. figs 5 & 6. 1828. p. 31, pi. 2. 

 In the foetal Ai examined by Dr. Harlan, these small deciduous teeth were, perhaps, not 

 developed. See this Author's Medical and Physical Researches, p. 549. 



(3) PI. 81, figs. 3, 4, 5. 



