ANNULOSA : CRUSTACEA. 253 



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 Carboniferous period. 



1. Cirripedia. The Cirripedes are hardly known as Palaeo- 

 zoic fossils, but valves of a singular member of this order (Tur- 

 rilepas) have been found in the Silurian Rocks of Scotland. 

 With this exception, 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 Eocene period, and attain- 

 ing their maximum in recent seas. The Verruddce. commence 

 in the Chalk, and the Lepadidce begin still lower, in the Juras- 

 sic Rocks, and attain their maximum of development in the 

 Cretaceous epoch. The Upper Silurian genus Tifrrilepas, 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 

 Lower Silurian 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 Upper Silurian, and Dithyrocaris 

 in the Carboniferous Limestone. All these forms, with other 

 similar ones, are believed to be most closely allied to the recent 

 Apus and Nebalia. The genus Estheria, represented by many 

 forms from the Devonian period to the present day, is also 

 probably to be referred here. 



4. Trilobita. The Trilobites are exculsively Palaeozoic fos- 

 sils. In the Upper Cambrian Rocks the so-called " primor- 

 dial zone" there occurs a singular group of Trilobites the 

 so-called primordial Trilobites distinguished by the posses- 

 sion of many larval characters. In the Lower and Upper 

 Silurian Rocks the Trilobites attain their maximum of develop- 

 ment. They are still well represented in the Devonian 

 Rocks; but they die out completely before the close of the 

 Carboniferous epoch, being represented in the Mountain Lime- 

 stone by three genera only (Phillipsia, Brachymetopus, and 

 Griffithides}. 



5. Eurypterida. These, like the last, are entirely Palaeozoic, 

 attaining their maximum in the Upper Silurian and Devonian 

 formations, and dying out in the Carboniferous Rocks. Ptery- 

 gotus, Eurypterus, and Slimonia are the most characteristic 

 genera. 



6. Xiphosura. The genus Limulus commenced, as far as is 



