146 PALEONTOLOGY 



showing signs of travel broken body-chamber, abraded 

 surface, or general damage, they may fairly be regarded 

 as having floated from a distance. 



In the process of burial on the sea-floor, the sediment 

 would fill the body-chamber, and penetrate through the 

 septal neck into the first gas-chamber, which it could 

 only fill up to the level of the entry. A little might 

 penetrate into the second chamber, but beyond that it 

 would have great difficulty in making its way unless 

 the shell was broken in places (though fine calcareous 

 mud is sometimes found in the air-chambers of apparently 

 unbroken shells). The empty chambers would rarely 

 remain, empty, however : water would permeate them 

 and deposit materials from solution. Thus in calcareous 

 rocks the chambers are usually filled or partly filled with 

 crystalline calcite ; in clays, with pyrite or marcasite. 

 Where very little sediment was being deposited, they 

 were often filled with calcium phosphate. As a conse- 

 quence, fossil cephalopods, especially those of large size, 

 are usually very heavy objects, and it needs an effort of 

 imagination to realize them as possibly floating animals. 

 Further, the thin shell itself may have been removed, in 

 one of two ways : by solution in the rock, or by flaking 

 away during extraction, so that we have an internal cast. 

 When the ornament is a corrugation, not a thickening, 

 of the shell, it shows as well on the cast as on the shell, 

 but the casts are at once distinguished by their showing 

 the septal sutures, which of course are on the inner, 

 not the outer, surface of the shell. 



Sometimes the shell itself may be replaced by silica or 



