2 GUIDE TO THE FOSSIL INVERTEBEATE ANIMALS. 



For the purposes of this Guide, it is more convenient to 

 take the collections in the reverse order from that given 

 above, namely, to begin with Gallery XI. The description 

 of the specimens exhibited in that Gallery serves to explain 

 the meaning of fossils and the use made of them in inter- 

 preting the structure and history of the earth. We tlien 

 proceed to Galleries X, VIII, and YII, beginning with the 

 simplest forms of life and passing gradually to those more 

 highly organised. Under each group of animals too, the 

 description generally begins with the older fossils and traces 

 the history of the group down to our own day. In order to 

 follow this method of description, it is, in Galleries X and 

 YIII, necessary to visit the Cases in the reverse order of 

 their numbering. 



The collections in Galleries X, VIII, and VII are 

 arranged, in the main, according to a zoological classification, 

 the specimens belonging to the various large groups of the 

 Animal Kingdom being placed together. In some groups it 

 has been found more convenient to subdivide the specimens 

 according to the geological epochs to which they belong, and 

 under each of those epochs again to arrange them in zoological 

 order. In other groups the zoological system is the dominant 

 one throughout, all species of each genus being placed 

 together. It is also the general rule that the specimens 

 from British localities are exhibited in the Table-cases, and 

 those from foreign localities in the Wall-cases. This, how- 

 ever, is not rigidly adhered to ; for example, among the 

 larger specimens mounted on blocks and placed usually 

 towards the back of the Wall -cases may often be found 

 several British specimens, which have to be exhibited there 

 on account of their size. 



Although all the animals whose fossil remains are herein 

 described are often spoken of collectively as Invertebrata, or 

 backbone-less animals, in contrast to the Vertebrata, or 

 animals with a backbone, yet it must be remembered that 

 this common usage does not represent a scientific classifica- 

 tion. It is nearer the truth to say that each of the larger 

 groups named above — Mollusca, Arthropoda, Echinoderma, 

 Coelentera, and the rest — has the same independence and 

 importance as a division of the Animal Kingdom as has the 

 whole group of Vertebrata. Just as the Vertebrata are 

 divided into Classes, namely, Mammals, Birds, Eeptiles, 

 Amphibians, and Fishes, so is each of these great groups, or 

 Phyla as they are termed, divided into Classes. Each Class 



