12 MAMMALS 



several deep pits, and also mark the sides with one or more grooves, and fill up 

 such pits and grooves with sealing wax, it is obvious that we shall have a much 

 more complex type of structure. This complex model will serve to explain the 

 type of tooth structure found in many of the Hoofed or Ungulate Mammals ; and it 

 will be obvious that if we now cut off the summit of our model we shall find a series 

 of irregular discs of beeswax (ivory), each surrounded by a sinuous border of kid 

 (enamel), in the folds of which will be masses of sealing wax (cement). Such a 

 model will enable us to understand the nature of the cheek-teeth of the Ungulate 

 Mammals when we come to them. 



Importance of From a utilitarian point of view Mammals are of extreme impor- 

 Mammals tance to man, since it is from them and more especially from the 

 to Man Ungulate order that by far the greater part of his animal food is 

 procured, while their skins or fur furnish him largely with raiment ; and it is from 

 their ranks alone that all his beasts of burden and draught are recruited. More- 

 over, since these creatures are the highest representatives of the animal kingdom, 

 among whom man himself must, from a zoological standpoint, be included, their 

 study is one which commends itself most forcibly to all who are in any way inter- 

 ested in Natural History. 



Mammals in Numerous as are the Mammals now living, it must never be for- 

 the Past gotten that they form but a small moiety of those which flourished at 

 earlier periods of the history of our earth. The Mammals of the present day may, 

 indeed, be compared to the topmost branches and twigs of a giant forest tree, of 

 which the larger limbs and trunk are concealed from our view. And it will accord- 

 ingly be manifest that any one who confines his studies to the existing species will 

 have but a very imperfect idea of the whole array of Mammalian life, and of the 

 mutual connection of its various branches. The study of fossil Mammals is, how- 

 ever, a difficult one, and one requiring an extensive knowledge of comparative 

 anatomy. All that can, therefore, be done in a work of the present nature is to 

 call attention, as occasion arises, to some of these extinct Mammals which are of 

 especial importance and interest as showing the manner in which groups now 

 widely separated from one another were formerly more or less completely con- 

 nected. 



Although the number of 

 extinct Mammals is very large, 

 yet by far the greater propor- 

 tion of these belong to the 

 latest of the three great epochs 

 into which the geological his- 



THE I.EFT HALF OF THE LOWER JAW OF AN EXTINCT 



POUCHED MAMMAL. **? of our ^o^ has been 



divided. Whereas, during the 



From the Cretaceous Rocks of North America. The tusk is , 



marked a. After Marsh. long-past epoch knOWU aS 



the Secondary period, during 



which our chalk and oolites were deposited, the earth was tenanted by gigantic 

 reptiles of strange form, it is not till we come to the rocks overlying the chalk, such 

 as the London clay and overlying strata, that we find Mammals taking an important 



