BOT 
from studies of the embryo of Loligo ; and these facts carry out his 
conclusions, substituting, however, the hood for the two mantle- 
flaps which were imagined by him as the organs which inclosed 
the shell and formed the shell-sac. 
Most paleontologists have considered the Sepioidea and Belem- 
noidea as more closely allied ; but they appear to us as two orders, 
certainly as distinct as, and perhaps even more widely divergent 
than, the Nautiloidea and Ammonoidea. 
Among these two orders we recognize many exceptional forms— 
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Fic. 2.—Argonauta sp. ? 
such as the Spirula among Belemnoidea, and among Sepioidea the 
octopods; and we think they all prove our position, that the 
habitat so closely accords with the structural changes of the type 
that its purely physical agency must be regarded as the efficient 
