CRUSTACEA. 149 



present day. Similarly, of the two sub-orders of the Merosto- 

 mata, the Eurypterida are confined to the earlier portion of the 

 Palaeozoic period, whilst the more highly organised and less 

 larval King-crabs (Xiphosura) 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 doubt- 

 fully so, and will not be considered here. 



TABULAR VIEW OF THE DIVISIONS OF THE CRUSTACEA. 



Sub-class I. EPIZOA (Haustellata). 

 Order I. Ichthyophthira.* 

 ,, 2. Rhizocephala* 



Sub-class II. CIRRIPEDIA. 



( Balanidse. 



Order 3. Thoracica. < Verrucidse. 



( Lepadidse. 

 ,, 4. Abdominalia* 

 ,, 5. Apoda* 



Sub-class III. ENTOMOSTRACA. 

 Order 6. Ostracoda. 

 7 . Copepoda* 

 ,, 8. Cladocera* 

 ,, 9. Phyllopoda. > Legion, Branchiopoda. 



,, 10. Trilobita. \ 



,, II. Merostomata. 



Sub-class IV. MALACOSTRACA. 



Division A. EDRIOPHTHALMATA. 

 Order 12. Lcemodipoda* 

 ,, 13. Isopoda. 

 ,, 14. Amphipoda. 



Division B. PODOPHTHALMATA. 

 Order 15. Stomapoda. 

 ,, 1 6. Decapoda. 



Tribe a. Macrura. 

 ,, b. Anomura. 

 ,, c. Brachyura. 



SUB-CLASS CIRRIPEDIA. 



Animal free when young, but permanently attached in the adult 

 condition to some foreign body by the anterior extremity of the 

 metamorphosed head. The visceral cavity of the adult protected 

 by a calcareous shell of several pieces, or by a coriaceous envelope. 

 Abdomen free. Thoracic segments usually carrying six pairs of 

 forked ciliated limbs. 



