LARVAL STAGES OF TRILOBITES 183 



the origin and significance of the acquired characters, and 

 thus to assign to each its true value. 



Antiquity of the Trilobites. 



The superlative age of the trilobites has been generally 

 recognized, and is too well known to require more than a 

 passing notice. Even in the earliest Cambrian they bear 

 evidence of great antiquity in their diversified form, their 

 larval modifications, and their polymerous cephalon and caudal 

 shield, all of which features show that trilobite phylogeny 

 must reach far back into pre-Cambrian times. 



Not only are the smallest species (Agnostus) found in the 

 Cambrian, but also many of the largest (Paradoxides). 

 There is a great range of variation in the number of free 

 thoracic segments, varying from two in Agnostus to twenty 

 in Paradoxides. The pygidium likewise shows extreme vari- 

 ation of from two to upward of ten ankylosed segments. 

 The eyes may be absent as in Agnostus and Microdiscus, or 

 very large as in Paradoxides, though both in this respect 

 and in the number of somites, free or fused, the Cambrian 

 genera are exceeded in later deposits. In ornamentation 

 and spiniform processes the Cambrian species show consider- 

 able development, though not as great as others since that 

 time. However, the wide variation they do present in this 

 particular indicates differentiation and specialization consider- 

 ably removed from the beginning of the trilobite phylum. 



The acquisition of distinct larval stages could only have 

 been reached through a long series of changes in ancestral 

 forms. The composition of the cephalon and caudal shield 

 indicates a derivation from some primitive form, probably 

 annelidan, in which, through adaptation to special require- 

 ments, certain polar segments became fused, forming very 

 distinct terminal body regions. Furthermore, the trilobites 

 are the only large division of the Arthropoda which has be- 

 come extinct. The Merostomata and Phyllocarida culmi- 

 nated a little later, though still represented by living species; 



