OF CENTRAL CANADA PART IV. 



285 



Tetrabranchiata : In this nearly extinct Order, the shell judging 

 from the single surviving genus, the Nautilus is external, and is 

 divided into numerous chambers by concave or slightly sinuated par- 

 titions, known as " septa. ; ' These are formed successively during 

 the growth of the cephalopod the animal inhabiting the last-formed 

 outer chamber. The septa are traversed by a long tube or " siphun- 

 cle," passing through all the chambers, and serving to keep the 

 earlier-formed portions of the shell in connection with the animal's 

 body. This tube or siphuncle varies in size, shape and position. In 

 some forms it is of narrow diameter ; in others, comparatively wide. 

 In some, again, of uniform, gradually tapering shape ; in others con- 

 tracted at regular intervals, in which case it is known as a " beaded " 



FIG. 227. 



Section of shell of Nautilus (N), and of Orthcxferat; (O and O'), shewing form and 

 position of septa and siphunele. 



siphuncle ; and as regards position it may be central or more or less 

 excentric. Worn sections of the orthoceras shell, especially when 

 the siphuncle is beaded and the outer portion of the shell is desti-oyed, 

 much resemble the vertebral column of a fish, and are often taken 

 for this by quarry men and others. With regard to external form, 

 the shell may either be convolute (as in the nautilus), or simply 

 curved or more or less conical. The concave edges of the septa 

 generally shew on the external surface of the shell ; but these must 

 not be confounded with the lines of growth, ribs, striae, and other 

 surface markings and ornamentation. The mouth or aperture of the 

 shell is commonly open or expanded, but in certain forms it is more 

 or less contracted, and in some few almost closed a narrow (some- 

 what key-hole shaped) opening only remaining. Merely the arms 

 or tentacles of the animal could have been protruded through this : 

 always supposing the shell to have been external. Occasionally, 

 in the larger orthoceratites especially, the posterior portion of the 

 siphuncle is filled with a solid conical secretion of calcic carbonate, 



