234 MANUAL OF ZOOLOGY. 



LepadidcR begin still lower, in the Jurassic Rocks, and attain 

 their maximum of development in the Cretaceous epoch. 



2. Oj^rawd^a.— -Small Ostracode Crustacea are exti'emely 

 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 Palasozoic Rocks. Hymenocaris 

 is found in the Upper Cambrian, Caryecaris 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 sxid Nebalia. .) j . 



4. Trilobita. — The Trilobites are exclusively 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 devel- 

 opment. 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 Linae- 

 stone by three genera only {PhUltpsia, 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,a.TiASlimoniaax& the most characteristic genera. 



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

 yet known, in the Permian period) arid has survived up to the 

 present day. Its first appearance, therefore, was, just at the 

 close of the Palasozoic epoch. The two remaining genera, 

 which constitute with Limttl^s this sub-order (viz., Bdinurus 

 and Presiwichia), are Palaeozoic, and are not known to occur 

 out of the Carboniferous Rocks. 



7. Isopoda. — The earliest known Isopod is the Prosoponiscus 

 of the Permian Rocks. 



8. Stomapoda. — This order is doubtfully represented in the 

 Carboniferous Rocks. 



9. Decapoda. — The Decapods, with the exception of a single 

 doubtful form from the Carboniferous Rocks, are not known to 

 have existed at all duHng the Palaeozoic period ; but they are 

 well represented, in all their three tribes, in the Secondary and 

 Tertiary epochs, attaining their maiximum at the present day. 

 The London Clay, (Eocene) is especially rich in the remains 

 of Macrura and Brachyura. 



