MAMMALIA. 399 
and the Didelphes, much less numerous, whose young come into the world 
at an epoch when they are not yet fully developed, and for that reason 
require more special and extraordinary care from the female, who is fur- 
nished with an abdominal pouch, where the young are sheltered, and at 
first permanently attached to the teat. The Didelphes have besides, two 
additional bones to their skeleton, called the marsupial bones. 
This was a decided improvement, but the Edentata still remain isolated 
or scattered, without permanent place. 
This fundamental division into two parallel groups corresponds exactly 
to another, based upon the presence or absence of a placenta, that part of 
the foetal envelopes which unites the young more intimately with the mother, 
and by means of which they are supplied with blood. The Monodelphes 
are called Placentalia, whilst the Didelphes are Aplacentalia, or deprived of 
this organic connexion. Such a division may appear natural at first sight, 
as it seems to express some embryonic law. But embryology has not yet 
done all that we may expect from it with regard to our classification. It 
will furnish us one day with data by which the natural affinities and 
mutual relationships of the fundamental, and also minor groups, shall be 
unfolded. The embryology of mammals is still in its infancy, especially in 
that part of the genesis which relates to the earliest phenomena which take 
place immediately after the fecundation of the eggs. : 
The disposition of the minor groups in the systems just alluded to varies 
among the various authors, showing how far this class is from being 
understood. Thus the following arrangement was published in France in 
1845. 
Sus-Cuass I. Priacenrania. 
Orper I. Primates: Families, Simiade, Lemuride, Tarside, Chiro- 
myide. 
Orper I]. Tarpigrava: Fam., Bradipodide. 
Orver III. Cuerrorrera: Fam., Galeopithecide, Pteropodide, Ves- 
pertilionide, Noctilionide, Vampyridze, Desmodide. 
Orper IV. Carnivora: A. Carnivora proper ; Fam., Cercoleptide, 
Viverride. B. Amphibia; Fam., Phocide, Triche- 
chide. C. Insectivora; Fam., Eupleride, Tupaiade, 
Gymnuride, Macroscelide, Soricide, Talpide, Erina- 
*  ceide. 
Orver V. Ropentia: Fam., Sciuride, Muride, Pseudostomide, 
Spalacide, Hystricide, Leporide, Cavide. 
Orper VI. Pacuyprermara: Fam., Hyracide, Elephantide, Tapiride, 
Rhinocerotidz, Hippopotamide, Suide, Equide. 
Orpver VII. Ruminanrra: Fam., Camelide, Antilopide. 
Orper VIII. Evenrara: Fam., Dasypodide, Myrmecophagide, 
Manide. 
Orver 1X. Sirenipta: Fam., Manatide, Halicoride, Rytinide. 
“Orpsr X. Cerracea: Fam., Delphinide, Physeteride, Balenide. 
603 
