OF FOSSIL MULTILOCULAR SHELLS. 421 



and sometimes the tube is found to have dwindled away and become 

 a mere thread. While, however, the siphuncle diminishes in size and 

 importance, the general shape of the shell and peculiar form of the 

 septa indicate an increased capacity for resisting pressure and support- 

 ing the weight of a high column of water. 



Perhaps, viewing the subject in this light, we may not be far wrong- 

 in assuming a natural ground of separation between these two families 

 of cephalopods, since the one appears to have a contrivance for enabling 

 it to swim freely in the ocean, and rise or sink at pleasure, while 

 in the other, there is only as it were the rudimentary appearance of 

 this contrivance; but, on the other hand, additional strength in its ha- 

 bitation, fitting it to dAvell moi-e at the bottom of the sea and at 

 considerable depths, and there to keep within necessary limits those 

 Crustacea and molluscs, which might otherwise, by their rapid increase, 

 have interfered with the established course of nature. 



In applying this theory, if it may be called so, to our new genus, 

 we must necessarily consider separately the group described by Count 

 Miinster as having a small siphuncle, and the species now introduced 

 to your notice. In the former, there seems to be a provision for 

 strength, without great power of locomotion ; for the septa seem less 

 simple than even in some goniatites, and the lobes must be supposed 

 to increase the resisting power. In the latter there are no lobes, but 

 the siphuncle being so much larger, we may reasonably suppose that 

 the extent of the inhabitant's power of altering readily its depth in the 

 water, must have been in a corresponding degree greater. 



The study of comparative anatomy introduces to our notice, in a 

 very striking manner, the strong resemblances in the structure of dif- 

 ferent animals, and the universal occurrence of what would seem rudi- 

 mentary aftempt.1 at higher and more complete organisation. Such, for 

 instance, are the rudimentary bones in the fins of swimming mammalia, 

 which correspond to the bones of the extremities in man ; and such 

 would seem to be the case in this siphuncle, sometimes very large, 

 then diminishing in size and importance, till it dwindles down to the 



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