352 



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 



FIG. 548 PEARLY NAUTILUS ; with the shell laid open ; t, tentacula ; e, funnel ; p, 

 foot ; m, 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 



