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from studies of the embryo of Loligo ; and these facts carry out his 

 conclusions, substituting, however, the hood for the two mantle- 

 flaps which werejmagined 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 — 



Fig. 2.—Argonauta sp. f 



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 



