. Elements of zoölogy : a textbook. Zoology. 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- phu
. Elements of zoölogy : a textbook. Zoology. 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). '. Please note that these images are extracted from scanned page images that may have been digitally enhanced for readability - coloration and appearance of these illustrations may not perfectly resemble the original Tenney, Sanborn, 1827-1877. New York : Scribner
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