TETRABRANCHIATA : 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 n.ame 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 pomfilins, Linn. Cut open to show the chambers and 

 siphuncle. One half. Pacific and Indian Oceans. 



, OR NAUTILUS FAMILY. This Family 

 contains a single living representative species, or perhaps 



