MALACOSTRA.CA 45 



spicuous than in most genera, are characteristic of the Lower 

 ►Silurian strata. Two others from the Wenlock limestone have 

 long been celebrated, viz., Calymcne (fig. 9, 9), or the " Dudley 

 Trilobite/' so compactly rolled up ; and Asaphus (or Phacops) 

 caudatus (fig. 9, 8), in which the lenses of the large eyes are 

 frequently well preserved, and visible without a glass. Each 

 eye has at least 400 facets, and in the great Asaphus tyrannus 

 each is computed to have 6000. In one species {Asaphus 

 Koicalcicskii) the eyes are supported on peduncles. The largest 

 Trilobite is Asaphus gigas ; some of the fragments indicate a 

 creature eighteen inches long. 



Sub- Class 2.— MALACOSTRACA. 



Char. — Body divided into thorax and abdomen, with seven 

 segments in each. 



The Isopods are represented in the upper oolite by Arcli- 

 ceoniscus Brodicei, which is gregarious, in large numbers in the 

 slabs of Purbeck limestone ; and in the Permian system by 

 the Prosoponiscus (or Palccocrangoii). The problematic Pygo- 

 cephalus, and the " Apus duhius? both from the carboniferous 

 strata, are doubtfully referred to the Stomapoda, and, with the 

 exception of the Gitocrangon of Eichter, are the oldest of the 

 known stalk-eyed Decapods. 



Macrourous Crustacea are of constant occurrence through- 

 out the oolites and cretaceous strata. One of the most remark- 

 able forms, Ergon (fig. 10, 3), is found in the lias (with the 

 closely-allied Tropifcr and Colcia) and in the Oxford clay. 

 The small lobsters of the genus Glyphea, in the oolites, and 

 Mcycria, in the Speeton clay and greensand, are commonly the 

 nucleus of hard nodules of phosphate of lime. The larger 

 species of the chalk form the genus Enoplodytia. The Oxfor- 

 dian oolite of Solenhofen, with its finely-laminated lithographic 

 slates, opens like a book filled with compressed and wonder- 



