or unvascular dentine, and a thick outer coating of cement. Tothese, of course, may be added the dental characters common to theorder Bruta : viz. uninterrupted growth of the teeth, and theirconcomitant implantation by a simple, deeply excavated base, notseparated by a cervix from the exposed summit or crown.

In the mature Ai , (Bradypus tridactylus, Linn.) the teeth(l) aresmall, of a simple columnar form, presenting, for the most part,a sub-elliptical transverse section, sub-equal, and separated fromeach other by short intervals. The grinding surface has a centraldepression, with a raised margin, worn unequally into one or twopoints. The first tooth in the upper jaw is the smallest, the secondis the largest; both approach the trihedral form. The first in thelower jaw is more curved than the rest, and is compressed frombefore backwards, the posterior surface forming a slight angle, andbevelled down obliquely from the anterior margin, which is trenchant,and sometimes notched: the fourth or last molar of the lower jawis subtetragonal, and rather larger than the rest. The teeth ofthe upper and lower jaw are not opposite; but subalternate whenthe mouth is closed.

The dental formula of the genus Bradypus is

Inc. jj, Canini l, Molares J-Ej = 18.

Dr. Brant(2) has described and figured the skull of a youngAi , in which a very small tooth preceded the compressed one oneach side of the lower jaw r , rendering the number of teeth equalto that in the upper jaw. The remains of the alveoli of these teethare visible in the jaw of a young Ai in the Hunterian Museum, butthey are soon shed ; and, if constant in the species, are confinedto the immature period.

In the tw’o-toed Sloth or Unau, (Cholrcpus didactylus, Illig)the teeth(3) offer a greater inequality of size than has yet beenobserved in any other genus of Bruta; the first of each series, inboth jaw’s, which in the rest of the Order is the smallest, here so

(1) PI. 81, fig. i.

(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, notdeveloped. See this Author’s Medical and Physical Researches, p. 549.

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