352 
PEOFESSOE OWEX ON THE FCETAE IMEIVIBEANES 
the Catarhines, I have ascertained that in the Chimpanzee {Troglodytes niger) the pla- 
centa is single, as in the Human subject. 
The true or horned Kuminants all present the much subdivided cotyledonal type of 
placenta ; but the Camel-tribe are unquestionably much more nearly aUied to the true 
Ruminants than to the Equine-tribe, which they resemble in the diffused vascular villo- 
sity of the chorion, which is substituted for a placenta. Thus that organ is not one of 
the characters which indicate the real affinities, and which guide to the true grouping, 
of the TJngulata, any more than it subserves to the formation of natural groups of the 
Quadrumana. The Hog, which has a diffused chorionic \illosity, as in the Camel, but 
with a modification in the arrangement of the vessels, which Su’ Etekaed Home defines 
by the term ‘ stellate structures is, like the Camel, much more nearly related to the 
true Ruminants than to the odd-toed hoofed animals, such as the Horse, Tapir and Rhi- 
noceros. So, likewise, the Hyrax, with an annular placenta, is more closely related to 
the Rhinoceros than to the Elephant or the Lion. 
I would not be misunderstood as deeming the placenta to present no correlation of 
form with modifications of mammalian structure : the constancy of the annular type in 
the Carnivora^ and that of the cotyledonal type in the true Muminantia, are valid instances 
of the degree to which that correlation exists. But the modifications of the placenta do 
not form a true guide to the affinities or classification of the Mammalia generally. The 
primary groups of that class are based upon modifications of a much more important 
and significant organ — the brain : and of this truth Sir Everaed Home himself appears 
to have been aware f, when he pubhshed the placentary system of classification, which 
has lately been revived, with modifications, by the distinguished Professor in the Jai'din 
des Plantes J. 
The placenta of the phytiphagous Sloth {Bradypus)^ which has a complex subdiHded 
stomach, is almost as much subdivided as in the smaller Ruminants ; whilst the placenta 
of the Armadillo, which has a simple stomach, is a single tliin oblong body, as in the 
Hedgehog. But the true affinities of the Sloth would be as much violated by tiuns- 
ferring it to the Ruminantia, as those of the Armadillo would be by classing it vsith the 
Insectivora, or as those of the Hyrax are by its transference to the same primary group 
with Carnivora, on the score of mere accordance of placental form. 
A very significant evidence of the limited correlation of any particular form of pla- 
centa with any of the known modifications of the mammahan t)q)e, is afforded by the 
* See Escheicht, ‘ De organis quse respirationi et nutritioni foetus mammalium inserviunt,’ ■4to, 1837, 
p. 4 ; and the description of the venous plexules of the chorion, in ‘ Catalogue of the Physiological Series, 
Mus. College of Surgeons,’ vol. v. p. 134, No. 3543. 
t “ The truth becomes obvious of there being no organ belonging to an animal, except the brain, that 
will bear us out in aifording characters for a general classification, the sti’ucture of the other organs being 
varied whenever it was necessary to adapt the animal to the climate which it is to inhabit, or the food on 
which it is to subsist: and the brain we are not sufficiently acquainted with, to take it as our guide.” — 
Lectures on Comparative Anatomy, vol. iii. p. 445. 
J Milxe-Edwaeus, loo. cit. Annales des Sciences NatureUes, ser. 3. vol. v. 1844. 
