424 THE VERTEBRATE SKELETON. 



gap from the molars, which have their crowns marked by ridges 

 or cusps. There are indications of several vestigial incisors. 



Coenolestes, a remarkable form recently described from 

 America, belongs to the diprotodont section, and is the only 

 living member of the section known outside the Australian 

 region 1 . An exceptional dentition is seen in the case of the 

 extinct Thylacoleo, in which the functional teeth are reduced 

 to two pairs; one pair of large cutting incisors and one of 

 compressed sharp-edged premolars. 



EDENTATA. Some Edentata, viz. the anteaters (Myrme- 

 cophagidae) are, as far as is known, absolutely toothless at all 

 stages of their existence ; being the only mammals except 

 Echidna in which no tooth germs have been discovered ; 

 others, viz. the Manidae, though showing foetal tooth germs, 

 are quite toothless in post-foetal life ; others, viz. some of 

 the armadillos, have the largest number of teeth met with in 

 land mammals. The teeth are homodont except in the Aard 

 Yarks, and grow from persistent pulps. In the sloths (Brady- 

 podidae) and the Megatheriidae, there are five pairs of teeth in 

 the upper and four in the lower jaw. The teeth of sloths consist 

 of a central axis of vasodentine, surrounded firstly by a thin coat- 

 ing of hard dentine, and secondly by a thick coating of cement. 



In no living Edentate have the teeth any enamel; it has, 

 however, been described as occurring in certain early Mega- 

 theroid forms from S. America 2 , and an enamel organ has 

 also been discovered in an embryo Dasypus*. In the Arma- 

 dillos (Dasypodidae) the number of teeth varies from f or \ in 

 Tatusia, to upwards of f J- in Priodon, which therefore may have 

 upwards of a hundred teeth, the largest number met with in any 

 land mammal. In Tatusia all the teeth except the last are 



1 0. Thomas, P. Z. S., 1895, p. 870. 



2 .Ameghino,Bull.Ac. Argen.xu. p. 437. According to H. Burmeister, 

 Annal. Mus. Buenos Aires, in. 401 (1891), enamel does not occur, osteo- 

 dentine having been mistaken for it. 



3 E. Ballowitz, Arch. Mikr. Anat. XL. p. 133. 



