654 MANUAL OF ZOOLOGY. 



the Stonesfield Slate another interval succeeds, in which no 

 Mammalian remains have hitherto been found; but in the 

 fresh-water formation of the Middle Purbeck at the top, 

 namely, of the Oolitic series as many as fourteen small Mam- 

 mals have been discovered. These constitute the genera 

 Plagiaulax, Spalacotherium, Triconodon, and Galestes. In the 

 Jurassic rocks of North America, as in those of Europe, 

 small Mammals (belonging to the Marsupials, and referable 

 to the family of the Didelphida] have been discovered. An- 

 other gap then follows, no Mammal having hitherto been dis- 

 covered in any portion of the Cretaceous series (with doubtful 

 exceptions). 



Leaving the Mesozoic and entering upon the Kainozoic 

 period, remains of Mammals are never absent from any of the 

 geological formations. From the base of the Eocene rocks 

 up to the present day remains of Mammals commonly occur, 

 constantly increasing in number and importance, till we arrive 

 at the fauna now in existence upon the globe. 



The more important forms of fossil Mammals will be spoken 

 of in treating of the separate Mammalian orders. 



CLASSIFICATION OF THE MAMMALIA. Whilst there exists 

 little divergence of opinion as to the orders into which the 

 Mammalia may be divided, different authorities adopt different 

 views as to the great primary divisions of the class. Here it 

 will be sufficient to mention two modes of subdividing the 

 Mammalia, of which the first will be accepted as sufficient for 

 practical purposes. 



I. The Mammalia may be divided into two great primary 

 divisions, according as the structure known as the " placenta " 

 is present or absent. The "placenta" or "afterbirth" is a 

 highly vascular organ which is developed upon the exterior of 

 the envelopes of the foetus, and which is so closely connected 

 with the inner wall of the uterus as to allow of an interchange 

 of material between the blood of the embryo and that of the 

 mother.* The " Placental " Mammals are thus enabled to 

 carry their young for a much longer period than are the 

 " Implacental " Mammals, and hence the young animal in the 

 former is born in a much more perfectly developed condition 

 than in the latter. The subclass " Implacentalia," in which 

 there is no placenta, comprises only the orders of the Mono- 



* No traces of vascular prominences comparable to the Mammalian pla- 

 centa occur in any animals below the rank of Mammals, except in some 

 Sharks and some Ascidians. In the Sharks, however, the vascular emi- 

 nences are developed from the umbilical vesicle, and not, as in Mammals, 

 from the allantois. 



