I022 PRINCIPLES OF STRATIGRAPHY 



obtained. The wide distribution of the shell of Spirula is due to 

 the fact that after the death of the animal the shell ascends to the 

 surface, owing to the air-filled chambers, and then becomes a part 

 of the plankton. It is carried hither and thither by the currents 

 and waves, and finally may reach the sea-bottom in regions remote 

 from its original home, and be buried in sediments of every descrip- 

 tion, and under conditions in which the animal never existed. 

 Thus the shell of Spirula becomes an excellent index fossil, being 

 widely distributed and buried in all kinds of sediment. 



To a more restricted degree this method of pseudo-planktonic 

 distribution of the shell, after the death of the animal, occurs also 

 in Nautilus, the only modern representative of the tetrabranchiate 

 cephalopods. The animal belongs to the benthos, living in shallow 

 water in the tropics. Occasionally it swims near the surface, but 

 before long it returns to the bottom, where it crawls about with its 

 shell uppermost, feeding on Crustacea and other animals. On the 

 death of the animal the shell may float for a considerable time on 

 the surface, buoyed up by the air in the chambers, and thus it may 

 be carried for a greater or less distance before it settles to the 

 bottom, where it will be buried in all kinds of sediment (Walther). 

 The verity of such statements has. however, been questioned by 

 Ortmann and others, who believe that the Nautilus shell is seldom 

 carried far from the region inhabited by the living animal. 



What is true of the shells of Nautilus and Spirula is true of 

 the shell of Sepia, and was undoubtedly true of the shells of Am- 

 monites as well. (Walther-29 :5op; S^'-~5^ ^^ ^<^Q-) I'^ f^ct, we 

 may even believe that the shells of the Ammonites were better 

 floaters than either those of Spirula .or Nautilus, for these two 

 genera are retrosiphonate, the siphonal funnels passing backward 

 and thus giving more ready access to the water; while the shells of 

 the Ammonites were prosiphonate, their siphonal funnels bending 

 forward like the neck of a bottle, and thus making the entrance 

 of water more difficult. This conception of the planktonic wan- 

 derings of the shells of cephalopods after the death of the animal 

 furnishes a satisfactory explanation of many anomalies observed 

 in the occurrence of these animals in the geologic series. It ac- 

 counts, especially, for the sudden appearance and disappearance of 

 the same species in widely separated localities, irrespective of the 

 character of the rock, and of its normal faunal contents. This 

 widespread distribution of these shells makes them excellent index- 

 fossils, so that even small formations may readily be correlated by 

 their species of Ammonites, even though widely separated. 



It does not follow, of course, that ammonoid shells must always 



