196 MANUAL OP ZOOLOGY. 



1. Oirripedia. The Cirripedes are hardly known as Palseo- 

 zoic fossils, but valves of a singular member of this order 

 (Turrilepas) 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- 

 nidcB are the most common, commencing, as far as is yet 

 known, in the Eocene period, and attaining their maximum in 

 recent seas. The Verrucidce commence in the Chalk, and the 

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

 their maximum of development in the Cretaceous epoch. 



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. 



4. Trilobita. The Trilobites are exclusively Paleozoic 

 fossils. In the Upper Cambrian Rocks the so-called * pri- 

 mordial zone ' there occurs a singular group of Trilobites 

 the so-called primordial Trilobites distinguished by the pos- 

 session of many larval characters. In the Lower and Upper 

 Silurian Rocks the Trilobites attain their maximum of deve- 

 lopment. 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 (Pliillipsia, Brachymetopiis 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. 

 Pterygotus, Eurypterus, and Stimonia are the most character- 

 istic genera. 



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

 yet known, in the Permian period, and has survived up to the 

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

 close of the Palaeozoic Epoch. The two remaining genera, 

 which constitute with Limulus this sub-order (viz. Belinurus 

 and Prestlvichia), 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. 



