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 

 requh'e 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. 



Sub-Class I. Placentalia. 



Order I. Primates : Families, Simiadae, Lemuridae, Tarsidse, Chiro- 



myidee. 

 Order II. Tardigrada : Fam., Bradipodidse. 

 Order III. Cheiroptera : Fam., Galeopithecidae, Pteropodida?, Ves- 



pertilionidae, Noctilionidae, Vampyridee, Desmodidae. 

 Order IV. Carnivora : A. Carnivora proper ; /''am., Cercoleptidaa, 

 Viverridae. B. Amphibia ; Fam., Phocidne, Triche- 

 chidae. C. Insectivora ; Fam., Eupleridae, Tu[/aiadae, 

 Gymnuridae, Macroscelidae, Soricidse, Talpidae, Erina- 

 ceidae. 

 Order V. Rodentia : Fam., Sciuridae, Muridae, Pseudostomidae, 



Spalacidae, Hystricidae, Leporidae, Cavidae. 

 Order VI. Pachydermata : Fam., Hyracidae, Elephantidae, Tapiridae, 



Rhinocerotidae, Hippopotamidae, Suidae, Equidae. 

 Order VII. Ruminantia : Fam., Camelidae, Antilopidae. 

 Order VIII. Edentata : Fam., Dasypodidae, Myrmecophagidae, 



Manidae. 

 Order IX. Sirenidia : Fam., Manatidae, Halicoridae, Rytinidae. 

 Order X. Ce'tacea : Fam., Delphinidae, Physeteridae, Balaenidae. 



603 



