358 



concealed by the fleshy parts. A perfect specimen was 

 dredged off Grenada in the Caribbean by the "Blake,'' from 

 a depth of 950 fathoms.* 



Spirilla would seem to be a widely distribnted form, judg- 

 ing from the occurrence of its shell in almost all parts of the 

 tropical and temperate seas. The animal, however, is very 

 rare, only one specimen with soft tissues preserved having 

 been obtained by the ''Challenger" expedition, this being- 

 taken close to the island of Banda, in 360 fathoms ("Chal- 

 lenger"' Narrative). Altogether, perhaps, only about half a 

 dozen animals with the soft parts preserved have been ob- 

 tained. 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 description, and under 

 conditions under which the animal never existed. Thus 

 the shell of Spirula Avill become an excellent index-fossil, 

 being widely distributed and buried in all kinds of sedi- 

 ment. 



To a more restricted degree this method of distribution of 

 the shell, after the death of the animal, occurs also in Nauti- 

 lus, the only modern representative of the tetra branchiate 

 cephalopods. The animal belongs to the benthos, living in 

 shallow water in the tropics. Occasionally, it swims near 

 the sui-face, but before long it returns to the bottom, where 

 it crawls about with its shell uppermost, feeding on Crusta- 

 cea 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 will be carried to a 

 greater or less distance before it settles to the bottom, where 

 it Avill be buried in all kinds of sediment. 



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 



* Agassiz, '88, II., p. 61. 



