THE CEPHALOPODA 145 



which are mostly asymmetric and crawlers, but in 

 which the few swimming or floating forms become 

 symmetric, we may suppose these few asymmetric 

 cephalopods to have been crawlers (benthic), while the 

 rest were, at the least, not confined to the bottom, even 

 if they were not habitually swimmers (nektic) or floaters 

 (planktic). 



The thin shell with its large volume of gas-chambers 

 must have been a very light object in the animal's life- 

 time, enabling the animal to rise or sink in the sea with 

 ease, as does the modern Nautilus. After death it might 

 float and be carried by currents to a considerable distance. 

 Thus the three surviving species of Nautilus live, in the 

 Polynesian seas between Sumatra and Fiji, but their 

 empty shells are found on the coast of Japan and else- 

 where; the three species of Spirula live in deep sub- 

 tropical waters, and only about half a dozen specimens of 

 the animal have ever been found, but the empty shells are 

 found in great numbers on the coasts of New Zealand, 

 and are also known in the Banda Sea, the Canary Islands, 

 West Indies, etc., and have even been found on the 

 Cornish coast. Thus it may be the case with fossil 

 cephalopods that the geographical distribution of the 

 shells is far wider than that of the living animals. Mr. 

 Buckman points out, however, that in travelling long 

 distances such delicate shells could not escape injury; 

 so that when we find (as we frequently do) fossil cepha- 

 lopods with the most delicate structures preserved 

 uninjured, it is reasonable to suppose that they lived 

 where they are now found ; but when we find the shells 



10 



