102 PROFESSOR TURNER ON THE PLACENTATION OF THE SLOTHS. 
separated, affords room for much speculation and thought, and throws a new 
light, not only on the position of the sloths in the order Edentata, but on their 
relations generally to the placental mammals. 
, Professor H. Mitne-Epwarps, in the Memoir above referred to, argues that 
similarity in the form of the placenta and in the arrangement of the membranes 
is associated with resemblances in other important structural characters, so 
that the classification of mammals founded upon the placenta rests on a 
natural basis. * Thus Man, the Quadrumana, Cheiroptera, Insectivora, and 
Rodents, are grouped together by him in the Micrallantoid legion of the 
phalanx Hématogénétes, as they possess in common a discoid placenta, a small 
allantois, and a caduca uterina. But, further, they are all markedly unguica- 
lated, their teeth are provided with a covering of enamel, and the dental series 
is continued around the front of the jaws. 
As regards their placental characters, the Sloths would fall into this Micral- 
lantoid legion, with which also they would be associated by their long claws ; 
but in the structural characters of their teeth and the absence of incisors they 
are at once markedly distinguished from them, so that in these respects the 
correspondence between placental form and structure and these other well- 
pronounced natural characters breaks down. 
Between Choloepus and Homo the divergence in most of their organic systems 
is so great that it is difficult to find evidence of any affinity except in their 
placental characters. With the Prosimii and Apes, however, affinities may be 
found. Der BLAINVILLE had indeed many years ago* indicated correspondences 
in the skeleton of the Sloths and the Apes, more especially the Gibbons; I may 
here refer to the very remarkable vascular plexuses which exist in the limbs 
both of the Sloths and Lemurs; and now that I have called attention to the 
evidences of affinity with these higher mammals it is not improbable that other 
features of resemblance may in time be recognised. From the point of view of 
the descent hypothesis, it is possible that between the Sloths and the Lemurs 
genealogical relations may exist. 
In conclusion, I may state that the study of the placenta in the Sloth has 
shown how difficult it is to predicate from the arrangement and structure of 
the other organic systems what the character of the placenta may be, and how 
necessary it is, before a proper estimate can be formed of the nature of the 
placentation, not only that the form of the organ and the arrangement of the 
membranes in the different orders of the mammals should be worked out, but 
the modifications in its minute structure should also be determined. More- 
over, it would seem that affinities in placental form and structure may exist 
between mammals which in many other respects are widely separated, so that 
the placenta is not in itself sufficient to determine the position of an animal in 
* QOstéographie des Mammiftres. Paresseux, p. 1. 

