7 
ceans; in it there are but slight traces of 
antennae, and the shield which covers the an¬ 
terior portion of the body, is expanded entirely 
over a series of crustaceous legs. Beneath 
the second, or abdominal portion of the shell, 
is placed a series of thin, horny, transverse 
plates, supporting the fibres of the branchiae, 
and at the same time acting as paddles for 
swimming. The same disposition of laminated 
branchiae is found also in the serolis. Thus 
while the serolis presents a union of antennae 
and crustaceous legs, with soft paddles bear¬ 
ing the branchiae, we have in the limulus a 
similar disposition of legs and paddles, and 
only slight traces of antennae; in the branchi- 
puswe find antennae, but no crustaceous legs; 
while the trilobite being without antennae, and 
having all its legs represented by soft paddles, 
is by the latter condition placed near branchi- 
pus among the entomostracous crustaceans, 
in the order of branchiopods, whose feet are 
represented by ciliated paddles, combining 
the functions of respiration and natation. 
“ In the comparison here made between 
