INTRODUCTION. 



With the exception of one doubtful organism (the Eozoon Canadense) not 

 met with in the oldest known British Sedimentary rocks, the fossil repre- 

 sentatives of the class Crustacea take rank in antiquity amongst the earliest 

 known organic remains. 



From the recently discovered P/-e-Cambrian rocks of St. David's, Pem- 

 brokeshire (the "Dimetian" and "Pebidian" formations) no organic re- 

 mains of any kind have been obtained ; the Lower Cambrian series, how- 

 ever, have yielded to the labours of Mr. Henry Hicks, F.G.S., and others, 

 remains of Molluscoida, Annelida, and Crustacea, 



Of the thirteen Orders, enumerated in the Table at page viii, tiuo 

 only (printed in Italics) are extinct, namely the Triiobita and the Eurt- 

 riERiDA, and three are not represented in a fossil state, viz. the Cladocera, 

 the CoPEi'ODA, and the Khizocepilala. 



The small Table below is intended to show at a glance the earliest appear- 

 ance in time (so far as we have been enabled to ascertain it) of each order, 

 and its recurrence in each successive formation. 



