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ORDER II. TETRABRANCHIATA. 



894. From the remains preserved in a fossil state, the Cepha- 

 lopoda of this Order appear to have been formerly most abund- 

 ant in our seas ; as they present themselves throughout almost 

 all marine strata, from the very earliest of the Palaeozoic series, 

 to those of a comparatively recent epoch. Yet some causes, of 

 which we are at present ignorant, have produced the almost 

 entire extinction of the Order ; the only existing representative 

 of it being the Nautilus pompilius, or Pearly Nautilus, so named 



Fro. 548 NEARLY NAUTILUS ; with the shell laid open ; t, tentacula ; e, funnel ; p, 

 foot ; TO, portion of mantle ; o, eye ; g, siphon. 



from the nacreous lining of its shell. The shell of this animal 

 is well known ; being found on most shores between the tropics. 

 Of the animal which constructs it, however, Naturalists had, 

 until recently, the most vague and incorrect ideas, the* Nautilus 

 being very rarely met with in the living state, owing to its being 





