of the Order Edentata. 357 



fish,* very different from any modification in the teeth of other 

 Edentata, or of other Mammalia. The intimate structure of 

 the teeth of the Mcgatherioids and Sloths, is quite as peculiar 

 to them among mammalia ; but this modification has not been 

 observed in any other class of vertebrate animals. 



This stmctural peculiarity of the teeth, and their continual 

 growth in the Sloths, are characters which, independently of 

 the total absence of incisors, and diminished number of mo- 

 lars, form an essential objection against their approximation 

 to the quadrumanous order ;t and the value of these differen- 

 tial characters is greatly increased by their close repetition in 

 the teeth of all the large extinct Mogatherioid animals, which, 

 whilst essentially related to the existing Sloths in other parts 

 of their organization, approximate in those modifications by 

 which they differ from the Sloths, not to the Quadrumana, but 

 to the Ant-eaters, and in a minor degree to the Orycterope and 

 Armadillos ; thus demonstrating by their differences, as by their 

 resemblances, the essential relations of the Sloths to the Eden- 

 tate order of Mammalia. 



The degradation of the armature of the jaws in this order 

 produces, especially in the truly edentulous Ant-eaters, a re- 

 semblance to the class of birds in one of their best marked 

 characters ; and amongst the implacental Edentata, we find the 

 jaws themselves assuming the form of a duck's bill in the Or- 

 nithorynchus. 



It may be observed of the Sloths, that they illustrate this 

 affinity or tendency to the oviparous type by the supernu- 

 merary cervical vertebrae supporting false ribs, and by the 

 convolution of the windpipe in the thorax, in the three-toed 

 species ; by the lacertine character of three and twenty pairs 

 of ribs in the Unau ; and by the single excrementory or cloaca! 

 outlet, by the low cerebral development, by the great tenacity 

 of life and long enduring irritability of the muscular fibre, in 

 both species. J Most interesting, therefore, becomes the disco- 



* Report of the British Association, 1838, p. 145. 



t M. de Blainville admits, that the character of the dental system, — " Co 

 systeme dentaire plus ou moins incomplet," loc. cit. p. 58, — is an indication of 

 their affinity to the Edentata. 



\ " Cor motum suum validissime retinebat, postquam exemptum erat e corpore, 

 per semihorium ; — Exempto corde caeterisquc visceribus, multo post se movebat 



