TETUABItANCUIATA Oil FOI'R-GILLED CEl'HALOPOUS. 399 



SUB-SECTION III. 



THE ORDER OF TETRABRANCHIATA OR FOUR-GILLED 



CEPHALOPODS. 



THE Tetrabranchiata liave four branchine, and an 

 external chambered shell, and very numerous arms. 

 Their shell is an extremely elongated cone, and is straight, 

 or variously folded, or coiled, Flr 53<J 



and is divided into cham- 

 bers by partitions called sep- 

 ta, the animal as it grows 

 forming a wall behind itself 

 at regular intervals, and al- 

 ways living in tlio outer 

 chamber - - communicating, 

 however, by a tube or si- 

 phuncle with all the others. 



The Tetrabranchiates are 

 best known under the name 

 of Chambered Shells. Al- 

 though only a very few spe- Fussil chambered 

 cies are now living, more than fourteen hundred species 

 have been found fossil in rocks below the Tertiary, and 

 varying in size from an inch to a yard in diameter. 



In the Nautilidse the siphuncle is central, and the septa 

 simple (Fig. 540). 



In the Ammonitidae the siphuncle is dorsal, and the 

 septa are zig-zag, or folded in a very complicated manner 

 (Fig. 541). ' 



