Larval Staijea of Trilohifes. — Jiiechcr. \H\ 
Y. Antiquity of the Tkilobites. 
The superlative age of the trilobites lias been generally rec- 
ognized, 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 modifi- 
cations, and their polymerous head 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 found in the ('ambrian 
(Af/nostus), but also many of the largest {Parado.n'des). There 
is a great range of variation in the number of free thoracic 
segments, varying from two in A(jiH>sfi/s to twenty in Paradox- 
ides. The pygidium likewise shows extreme variation of from 
two to upwards of ten ankylosed segments. The eyes may be nh- 
%ew.t SLs'in Agnosfiis and Mierodiscns, or very large as in Para- 
doxides, though both in this respect and in the number of 
somites, free or fused, the Cambrian genera are exceeded in la- 
ter deposits. In ornamentation and spiniform processes, the 
Cambrian species show considerable development though not 
as great as others since that time. However, the wide varia- 
tions they do present in this particular indicates differentia- 
tion and specialization considerably removed from the begin- 
ning 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 comi)Osition 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 ])ody regions. Furthermore, the tribolites 
are the only large division of the Arthropoda which has 
"become extinct. The Merostomata and Phyllocarida, cul- 
minated a little later, though still represented by living 
species, but all the other divisions a])parently have continued 
to increase since their inception during I'aleozoic time. The 
only known arthropod contemporaries of the trilobitcs in the 
Cambrian are the Merostomata, Ostracoda, Phyllopoda, and 
Phyllocarida, all of the higher forms apparently having de- 
veloped since that time. A more graphic view of the geolog- 
ical range and distribution of the arthroi)ods is represented in 
the following table : 
