166 



extinct Megatherioid animals, which, whilst essentially related to the existing 

 Sloths in other parts of their organization, approximate in those modifications 

 by which they diiFer from the Sloths, not to the Quadrumana, but to the Ant- 

 eaters, and in a minor degree to the Orycterope and Armadillos ; thus de- 

 monstrating by their differences, as by their resemblances, the essential relations 

 of the Sloths to the Edentate order of Mammalia. 



The degradation of the armature of the jaws in this order produces, especially 

 in the truly edentulous Ant-eaters, a resemblance 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 Ornithorhynchus. 



It may be observed of the Sloths that they illustrate this affinity or tendency 

 to the oviparous type by the supernumerary 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 cloacal outlet, by the low cerebral 

 development, by the great tenacity of life and long-enduring irritability of the 

 muscular fibre, in both species*. Most interesting, therefore, becomes the dis- 

 cover}', that in one of the huge extinct Sloths another character, heretofore 

 deemed peculiar to the class of birds, should have been repeated, viz. the bony 

 confluence of the last dorsal and the lumbar vertebrae with the sacrum. All 

 these indications of a transition to a lower class harmonize with the Cuvierian 

 view of the zoological position of the Sloths, as members of one of the lowest 

 and most aberrant orders of MammaUa ; and all oppose themselves to the pro- 

 motion of the Sloths to the Primates, and to their separation from the terrestrial 

 Edentata, which afford in the Ant-eaters and Pangohns, the Echidna and Or- 

 nithorhynchus so many additional retrograde steps towards the Oviparous 

 classes. 



It would be tedious to reiterate the special and gradational affinities of the 



* " Cor motum suum validissime retinebat, postquam exemptum erat 6 corpore, per semihorium ; — 

 Exempto corde cseterisque visceribus, multo post se movebat et pedes lente contrahebat sicut dormitu- 

 riens solet." Pison, Hist. Bras. p. 322, quoted by Buffou, who well obsen-es, " par ces rapports, ce 

 quadrupede se rapprocbe non-seulement de la tortue, dent il a deja la lenteur, mais encore des autres 

 reptiles et de tous ceux qui n'ont pas un centre du sentiment unique et bien distinct." — loc. cit. p. 45. 

 The endowment of a persistent formative organ of the teeth indicates another property in the Mega- 

 therioid animals, by which they would resemble the cold-blooded Reptiles, viz. longevity. 



