60 CEPHALOPODA. 



hranous or calcareous tiihc, termed the " siphmicle." The arms 

 are numerous, and are devoid of stickers; the hranchice are fonr 

 in number, tivo on each side of the body ; the funnel docs not 

 form a coriiplete tuhe ; and there is no ink-bag. 



The Tetrabrancliiate Cephalopods have an enormous de- 

 velopment in past time, an immense number of species, 

 mostly belonging to extinct types, being known from the 

 Pal£eozoic rocks alone. In the Mesozoic rocks the members 

 of this order were almost equally abundant. In the Tertiary 

 rocks the order is reduced to the single genus Nautilus, 

 represented at the present day by tlie single species Nautilus 

 pompilius (the Pearly Nautilus), with some very nearly allied 

 forms. The palteontological importance of this order being 

 so great, it may be as well to preface the account of the 

 extinct forms by a short description of the structure of the 

 living Nautilus pompilius, as described by Professor Owen, 

 from the most perfect specimen wliich has as yet been obtained. 



The soft structures in the Pearly Xautilus may be divided 

 into a posterior, soft, membranous mass (mcfasoma), contain- 

 ing the viscera, and an anterior muscular division, comprising 

 the head {prosoma) ; the whole being contained in the outer- 

 most, capacious chamber (tlie body-chamber) of the shell, 

 from wliich the head can be protruded at will. The shell 

 itself (fig. 448) is involuted and many-chambered, the animal 

 being contained successively in each chamber, and retiring 

 from it as its size becomes sufficiently great to necessitate 

 the acquisition of more room. Each chamber, as the animal 

 retires from it, is walled off by a curved, nacreous septum ; 

 the communication between the chambers being still kept 

 up by a membranous tube or siphuncle, which opens at one 

 extremity into the pericardium, and is continued through the 

 entire length of the shell. The position of the siphuncle is 

 in the centre of each septum. 



Posteriorly the mantle of the iSTantilus is very thin, but 

 it is much thicker in front, and forms a thick fold or collar 

 surrounding the head and its appendages. From the sides of 

 the head spring a great number of muscular prehensile pro- 

 cesses or " arms," which are annulated, but are not provided 

 with cups or suckers. In the centre of the head is the 



