TRTLOBITA. 351 



Triassic, Jurassic, Cretaceous, and some Tertiary deposits ; 

 but they appear to have attained their maximum develop- 

 ment towards the close of the Triassic period. 



The genus Learn (fig. 205, B) is very nearly allied to 

 Estheria, and comprises small Bivalved Crustaceans, with 

 "dark, horny, sub -quadrate valves, ; obliquely ridged from 

 umbo to angles, and ornamented with distinct lines of growth 

 parallel with the border" (Jones). Leaia is a very widely 

 distributed genus, but all the known species belong to either 

 the Carboniferous or Permian rocks. It has been suggested, 

 however, that the obscure Silurian genera Myocaris and Rib- 

 eiria are allied to Leaia. 



Lastly, we may notice that a Phyllopod nearly allied to 

 the living fresh- water genus Brancfiipus (fig. 202, D) has 

 been detected by Dr Henry Woodward in the Eocene forma- 

 tion, and has been described by him under the name Branchi- 

 podites vectensis. The much older Branchipusites (rightly 

 Branchipodites) anthracinus, of the Coal-measures, has been 

 supposed to have similar relationships, but this cannot be 

 said to -tie certainly established. 



ORDER TRILOBITA. 



Crustaceans in which the body is usually more or less dis- 

 tinctly trilobed ; there is a cephalic shield, usually bearing a 

 pair of sessile compound eyes ; the thoracic somites are movable 

 upon one another, and are very variable in number ; ike ab- 

 dominal segments are coalescent, and form a caudal shield ; 

 tJiere is a well-developed upper lip or " hypostome." 



As regards the general structure of the Trilobites, the body 

 was protected by a well -developed chitinous shell or " crust," 

 which covered the whole dorsal surface of the body, and 

 which usually exhibits more or less markedly a division into 

 three longitudinal lobes (fig. 206), from which the name of 

 the order is derived. In some cases, however, as in the 

 genera Homalonotus and Illcenus, this trilobation is only 

 obscurely marked. Though closely agreeing in their fun- 

 damental features, Barrande observes that it is not un- 



