ii PREFACE. 



and Stylonurus scoticus, obtained by James Powrie, Esq., F.G.S., of Reswallie, 

 Forfar. 



In the Upper Silurian we have one English genus, Hemiaspis, and three 

 Russian forms, Exapinurus, Pseudoniscus, and Bunodes, which, like the Decapoda- 

 Anomoura, serve partially to bridge over the interval between the brachyuran 

 Limuli and the macrouran Pterygoti, the segments of the hind-body being partially 

 developed. 



The Euryptepjda are eminently larval in their characteristics, and recall to 

 mind the earliest stages in the development of the living Brachyuran Decapod 

 (e. g. Carcinus mcenas), in which the principal locomotory organs are the 

 maxillipeds, the branchias (if developed) being external and thoracic, the abdominal 

 somites in both being destitute of appendages. Like the larva of these higher 

 forms, too, they have doubtless undergone further modification, and so in the 

 course of ages this primitive type has disappeared. The latest representative of 

 this extinct order has been found in the Lower Carboniferous series of West 

 Lothian, the Eurypterus Scoulcri, which, however, differs greatly from all the 

 other earlier forms. 



The brachyuran type, or Xiphosdra, are represented by a very minute form 

 with free thoracico-abdominal somites in the Upper Silurian of Lesmahagow, 

 Lanarkshire, the Neolimulus falcatus, and by numerous species in the Coal- 

 measures of Great Britain and Ireland, the United States, Germany, and 

 Bohemia. Some of these forms belonging to the genus Bellinurus had free 

 thoracic somites, and others, like Prestwichia rotundata, forcibly remind one of 

 the free-swimming larvas of the modern King-Crab. In the Oolitic period the 

 Limuli seem to have already attained to the greatest development of which they 

 were capable, and to be as highly differentiated as their modern representatives, 

 Limulus polyphemus and L. moluccanus. 



The Osteacoda, Phyllopoda, and Xiphosuea are good examples of persistent 

 types of Crustacea. They are orders the members of which have branched out 

 long since into bye-ways of their own, where, being checked from further 

 progress, they have by their great tenacity of life and large powers of reproduc- 

 tion held their ground through the long lapse of ages from Cambro- Silurian 

 times to the present day, whilst higher orders have been modified or swept away. 



H. W. 



117, Beaufort Steeet, Chelsea, 

 London, S.W. ; 



21st February, 1878. 



