CRUSTACEA. 333 



been a marked progression in proceeding from the older 

 formations to the present day. The Trilobites and Euryp- 

 terids of the older Palaeozoic rocks, though highly organised 

 so far as their type is concerned, are in many respects 

 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 repre- 

 sented at all till we reach the Carboniferous 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 present day. Similarly, of the 

 two sub-orders of the Merostomata, the Eurypterida, are con- 

 fined to the earlier portion of the Palaeozoic period, whilst 

 the more highly organised and less larval King-crabs (Xiplio- 

 mra), with few exceptions, did not make their appearance till 

 the Eurypterids had disappeared, at the close of the Car- 

 boniferous period. 



The following table shows the orders of the Crustacea, and 

 a short account will be given of the distribution in time of 

 those which are known to occur as fossils. The structure 

 also of the extinct groups will be shortly described. The 

 orders marked with an asterisk do not occur as fossils, or 

 only doubtfully so, and will not be considered here. 



TABULAR VIEW OF THE DIVISIONS OF THE CRUSTACEA. 



Sub-class I. EPIZOA (Haustellatd). 

 Order 1. Ichthyophthira.* 

 2. Rhizocephala.* 



Sub-class II. CIRRIPEDIA. 



C Balanidae. 



Order 3. Thoracica. \ Verrucidse. 

 ' Lepadidee. 

 4. Abdominalia* 

 5. Apoda.* 



Sub-class III. ENTOMOSTRACA. 



