SECT. I OBJECT AND LINE OF ARGUMENT 9 



run, proved unsuccessful, for reasons which we shall 

 try to point out. 



After briefly discussing the Eurypteridae, we shall 

 give an outline sketch of a new classification of the 

 Crustacea based upon our theory, showing that 

 while only one group of modern Crustacea admits of 

 derivation from the Trilobites, all the rest, except 

 Lirnulus, can be deduced from the Apodidae. We 

 shall see reasons- for believing that it was the develop- 

 ment of the shield, either as bivalve shell, or as a large 

 fold of the tergum of the fifth segment, which led to 

 success in the struggle for existence. 



We should here say something as to the preservation 

 of the Apodidae through so many geological ages. 

 This is explained by the manner of life of the animals. 

 They usually appear in ditches and pools dependent 

 on the rainfall. In such waters they naturally come 

 little into competition with other animals. The dry 

 seasons are bridged over by the eggs being preserved 

 in the mud. In this strange but perfectly natural 

 way, Apus has, from the earliest times, been so com- 

 pletely isolated that its preservation presents no 

 difficulty. Its presence in every part of the globe, 

 with almost always the same manner of life, is a sign 

 of its great antiquity. 



The fact that no true fossil Apodidae are found, 

 among the rich yield of Crustacean remains of the 

 Silurian strata, admits of simple explanation. We say 

 no true Apodidae, for we shall find that such forms as 

 Hymenocaris and Ceratiocaris, though perhaps some- 

 what more specialised, were probably very closely 



