TETRABRANCHIATA I NAUTILID^. 



475 



SUB-SECTION II. 



THE ORDER OF TETRABRANCHIATA, OR FOUR-GILLED CEPHALO- 



PODS. 



THIS Order comprises cephalopods which have four 

 branchiae, an external chambered shell, eyes peduncu- 

 lated, mandibles calcareous, and arms very numerous. 

 The shell is an extremely elongated cone, and is straight 

 or variously folded or coiled, and is divided into cham- 

 bers by partitions called septa, the animal as it grows 

 forming a wall behind itself at regular intervals, and 

 always living in the outer chamber, communicating, how- 

 ever, by a tube or siphuncle with all the others. The 

 Tetrabranchiates are best known under the name of Cham- 

 bered Shells. Although but one or two species are now 

 living, more than fourteen hundred species have been 

 found imbedded in the rocks. 



Fig. 379- 



Pearly Nautilus, Nautilus pompilius, Linn. Cut open to show the chambers and 

 siphuncle. One half. Pacific and Indian Oceans. 



NAUTILID^:, OR NAUTILUS FAMILY. This Family 

 contains a single living representative species, or perhaps 



