CRUSTACEA. 331 



Strata more ancient than the eocene. In the chalk formation, the 

 highest form of Crustacean would be classed with the Anomoura ; 

 but the Macroura are the prevalent forms. Lobsters, crawfish, 

 shrimps, and otiier Macroura, but all of species now extinct, are met 

 with fossilised in the secondary strata, from the chalk to the coal 

 formation ; but below the chalk, there are no higher forms of Crus- 

 tacean than the Macroura. When we come to the coal-measures, the 

 Malacostraca disappear ; but we then find the gigantic Entomostracan 

 called the King-Crab {Limulus) ; thence down to the lowest strata in 

 which any trace of animal life has been found, the Crustaceous class 

 is exclusively represented by Entomostracous species, and more 

 particularly by a peculiar form of Entomostraca, which became 

 extinct before the coal-measures were deposited. Almost endless, 

 however, are the varieties under which this form was manifested in 

 the Palfeozoic periods : the term " trilobite " is given to the funda- 

 mental type or form, on account of the segments of the body being 

 divided into three lobes. 



Our countryman Lhwyd, or Lloyd, first discovered this fossil in 

 1699, and he gave it the name of Trinucleus. At that time zoo- 

 logical science was not sufficiently advanced to afford a definite 

 binomial sign for the extinct animal ; and Lloyd's name rather indi- 

 cated the petrifaction as such, than a species of the animal kingdom. 

 The idea of the place of the Trilobite in that kingdom began first to 

 be mooted about the time of Linnaeus, by whom it was consigned to 

 the class of insects, with the name of Entomolithus paradoxus. Knorr 

 next studied it, and gave it the name of " Trilobite," believing it to 

 be allied to the molluscous Chitons, which opinion prevailed to the 

 time of the Entomologist Latreille, and died with him. The better 

 preserved fossils, in fact, gave evidence of distinct organs of vision, 

 large compound eyes, with a cornea divided into numerous segments, 

 thus proving that the MoUusca could not be the primary division of 

 animals to which the Trilobites belonged ; and since that time all 

 naturalists have agreed in referring these fossils to the Crustacea. 



With regard to their nearer affinities, all who had studied the 

 Trilobites up to 1843, were of opinion that they were allied to the 

 higher, or !Malacostracous, division of the Crustacea ; and Mr. 

 Macleay, in his valuable appendix to " Murchison on the Silurian 

 Strata," places the Trilobites in a distinct order between the Isopoda 

 and Aspidophora. He based his views on the trilobed character of 

 the segments in Scrolls and Bopyrus, and showed that the eyes were 

 large, sessile, and compound in Cymothoa, as in Trilobites. The 

 Cymoihoa and other Isopods roll themselves up into balls, as we 



