TKILOBITES. 261 



close the valves entirely, and protrude tlie feet and antennae 

 at pleasure. They are microscopic beings, and abound in the 

 sea and in fresh water. We know five fossil genera, of which 

 two, Cypridella and Cyprella, are special to the carboniferous 

 stage. CytJierina is found in the Silurian rocks, Oypridina 

 commenced in the carboniferous, and Cypris in the wealden 

 stage. We find species of these three latter genera in our 

 present seas, lakes, and marshes. The remains of Cypris, a 

 lacustrine genus, are so abundant in the wealden of Kent 

 and of the Isle of Wight, that some of the clays of this 

 stage have a foliated character, each lamina thereof being 

 entirely covered with their bivalve shells. The accom- 

 panying figures represent three species common in the 

 wealden of Saiidown and Brook Bays, Isle of Wight 

 (fig. 174). 



Order TRTLOBITES (or Paleades, Dalman) Had the 

 carapace composed of several rings, divided into three lobes 

 by two lateral depressions. It is not easy to decide at what 

 ring the thorax terminates and the abdomen commences. 

 Burmeister thought that we ought to name all the free 

 segments up to the last cephalo-thoracic, and those concealed 

 under the latter abdominal rings. The number of the tho- 

 racic segments varies from five to twenty ; they present many 

 intermediate numbers in the different families. 



The lateral lobes of the anterior or cephalic segment 

 support the eyes, which are prominent, kidney-shaped, and 

 compound, and are often preserved in a high state of per- 

 fection (fig. 175). 



The mouth is imperfectly known, and we have not found 



FIG. 175. The compound Eye La Trilobite. 



