TRILOBITES AND OTHER FOSSIL CRUSTACEA. 181 



be placed much higher among the Crustacea than we 

 find them in Haeckel's " Systematic Survey." In 

 many species of Trilobites the empty eye-sockets can 

 be seen with the naked eye, notably so in Asaphus 

 caudatus, in which each eye contained four hundred 

 facets. According to Owen, Asaphus tyrannus pos- 

 sessed no fewer than six thousand eyes ! The number 

 of eyes among the Trilobites varies considerably ; 

 some specimens have none at all. 



I have already referred to the fact that the Tri- 

 lobites are peculiar to the Primary rocks. Although 

 they seem to range as high as the Permian, they are 

 chiefly confined to the strata below and including the 

 Carboniferous limestone. No fewer than four hundred 

 species, grouped in fifty genera, have been described 

 from these formations, and new forms are still occa- 

 sionally met with. The greater number of the species 

 are of a Silurian age ; those of the Devonian rocks 

 are of a well-defined character ; and those from the 

 Carboniferous limestone even more distinct still. It 

 would seem as if they reached their maximum of size, 

 as well as of variation, during the Silurian period. 

 The largest are Asaphus gigas y eighteen inches in 

 length, found at Llandeilo ; and Paradoxides, two feet 

 long. On the other hand, they appear to have 

 decreased in size as well as in numbers when we 

 reach the Carboniferous rocks. The genus Phillipsia, 

 there represented, rarely includes specimens more 

 than three-quarters of an inch in length. It ought to 



