A REVIEW OE THE TRILOBITES. 



281 



The trilobites appear at the base of the Silurian formations, and 

 die out in the lower Carbonifeixms. They constitute the only order 

 of crustaceans of which we have no living representatives. 



The phyllopods appear also at the base of the Silurians. The lo- 

 phyropods (cyprids, &c.) follow the same law. The cirrhopods com- 

 mence their existence with the lias — at least, if the doubtful bostri- 

 chopus be excluded from the group. Of the siphonostoma, no fossil 

 representatives are known. 



These geological relations are presented at a single view in the 

 accompanying scheme : — * 



7. Classification. The arrangement of the trilobites in natural 

 groups is beset with considerable difficulty. This arises in part from 



e fragmentary condition in which so many species are commonly 

 met with ; and partly from our still imperfect knowledge of the true 

 value of the various organization characters on which we are obliged 

 to base our collocations. Owing to the first source of error, it has 

 often happened that distinct types have been formed into a single 

 species ; whilst on the other hand, imperfect specimens of one and 

 the same species have been referred to even different genera. A sys- 



* All the fossiliferous rocks below the Devonian group are ban included in the term Silu- 

 rian. The Lotihyrojioda should he drawn down t<> the Lovwi hoi izontft] line. 



