ANNULOSA: CRUSTACEA. 313 



casting their shell, a single individual may leave repeated 

 traces of himself, and the number of fossils may considerably 

 exceed that of the individuals which actually underwent fossili- 

 sation. The Crustaceans appear to have commenced their 

 existence in the Cambrian period, remains of members of this 

 class being tolerably abundant in the higher portion of this 

 formation. The Palaeozoic formations, taken as a whole, are 

 characterised by the predominance of the orders Trilobita, 

 Eurypterida, Ostracoda, and Phyllopoda, of which the two 

 former are exclusively confined to this period. All the other 

 orders of Crustacea which have left any traces of their past 

 existence at all, appear to have come into existence before the 

 close of the Palaeozoic period. Upon the whole, however, 

 there has been a marked progression in proceeding from the 

 older formations to the present day. The Trilobites and 

 Eurypterids of the older Palaeozoic rocks, though highly 

 organised so far as their type is concerned, are in many re- 

 spects inferior to later forms, whilst they present some striking 

 points of resemblance to the larval forms of the higher groups. 

 The great group of the Stalk-eyed Crustaceans undoubtedly 

 the highest of the entire class is not represented at all till we 

 reach the Devonian rocks; and it is not till we come into 

 the Secondary period that we find any great development of 

 this group, whilst its abundance increases to a marked extent 

 in the Tertiary period, and it attains its maximum at the pres- 

 ent day. Similarly, of the two orders of the Merostomata, the 

 Eurypterida are confined to the earlier portion of the Palaeozoic 

 period, whilst the more highly organised and less larval King- 

 crabs (Xiphosura) hardly made their appearance till the Eu- 

 rypterids had disappeared, at the close of the Carboniferous 

 period. 



1. Cirripedia. The Cirripedes are hardly known as Palaeozoic fossils, 

 but valves of a singular member of this order ( Turrilepas] have been found 

 in the Silurian rocks. With few exceptions, the Cirripedes are entirely 

 confined in past time to the Secondary and Tertiary epochs. The Bala- 

 nidce are the most common, commencing, with the doubtful exception of a 

 Liassic form, in the Chalk, and attaining their maximum in recent seas. 

 The Verrucidce commence in the Chalk, and the Lepadidce, with one or 

 two exceptions, begin in the Jurassic rocks, and attain their maximum of 

 development in the Cretaceous epoch. The Upper Silurian genus Tur- 

 rilepas, above mentioned, is also referable to the Lepadoids. 



2. Ostracoda. Small Ostracode Crustacea are extremely abundant as 

 fossils in many formations, and extend from the Cambrian period up to 

 the present day. 



3. Phyllopoda. Remains of Crustaceans supposed to belong to this 

 order are found in the Palaeozoic rocks. Hymenocaris is found in the 

 Upper Cambrian, Caryocaris in the Lower Silurian, Ceratiocaris in the 



