8 
four different families of crustaceans, for the 
purpose of illustrating the history of the long 
extinct trilobites, by the analogies we find in 
the serolis, limulus, and brahchipus; we have 
a beautiful example, taken from the extreme 
points of time of which geology takes cogni¬ 
zance, of that systematic and uniform arrange¬ 
ment of the animal kingdom, under which 
every family is nearly connected with adja¬ 
cent and cognate families. Three of the 
families under consideration are among the 
present inhabitants of the water, while the 
fourth has been long extinct, and occurs only 
in a fossil state. When we see the most an¬ 
cient trilobites thus placed in immediate con¬ 
tact with our living crustaceans, we cannot 
but recognise them as forming part and par¬ 
cel of one great system of creation, connected 
through its whole extent by perfect unity of 
design, and sustained in its minutest parts by 
uninterrupted harmonies of organization. 
“ We have in the trilobites an example of 
that peculiar, and, as it is sometimes called, 
rudimentary development of the organs of 
