9 
locomotion in the class crustaceans, whereby 
the legs are made subservient to the double 
functions of paddles and lungs. The advocate 
for the theory of the derivation of existing 
more perfect species, by successive changes 
from more simple ancient forms, might ima¬ 
gine that he sees in the trilobite the extinct 
parent stock, from which, by a series of de¬ 
velopments, consecutive forms of more perfect 
crustaceans may, during the lapse of ages, 
have been derived; but according to this 
hypothesis, we ought no longer to find the 
same simple condition as that of the trilobite 
still retained in the living branchipus, nor 
should the primeval form of limulus have pos¬ 
sessed such an intermediate character, or have 
remained unadvanced in the scale of organiza¬ 
tion, from its first appearance in the carbo¬ 
niferous series, through the midway periods 
of the secondary formations, unto the present 
hour. 
“ Besides the above analogies between the 
trilobites and certain forms of living crusta¬ 
ceans, there remains a still more important 
