TETRABRANCHIATE CEPHALOPODS. 2// 



Of the Dibranchiate Cephalopods the record is less perfect, 

 as they have few structures which are capable of preservation. 

 They attain their maximum, as fossils, shortly after their first 

 appearance in the Secondary Rocks, where they are represented 

 by the large and important family of the Belemnitidce. Some 

 of the Teuthida and Sepiadce are found both in the Secondary 

 and in the Tertiary Rocks, and two species of Argonaut have 

 been discovered in the later Tertiaries. No example of a Di- 

 branchiate Cephalopod is known from the Palaeozoic deposits, 

 and the order attains its maximum at the present day. 



CHAPTER XXV. 

 TETRABRANCHIA TE CEPHALOPODS. 



THE Tetrabranchiate Cephalopods are characterised by being 

 creeping animals, protected by an external, many-chambered shell, 

 the septa between the chambers of which are perforated by a mem- 

 branous or calcareous tube, termed the " siphuncle" The arms 

 are numerous, and are devoid of suckers ; the. branchia are four 

 in number, two on each side of the body ; the funnel does not form 

 a complete tube ; and there is no ink-bag. 



The Tetrabranchiate Cephalopods have an enormous de- 

 velopment in past time, several thousand species, mostly 

 belonging to extinct types, being known from the Palaeozoic 

 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 the single species Naiitihis pompilius 

 (the Pearly Nautilus). The palseontological 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 only perfect specimen which has as yet been obtained. 



The soft structures in the Pearly Nautilus may be divided 

 into a posterior, soft, membranous mass (metasomd), containing 

 the viscera, and an anterior muscular division, comprising the 

 head (prosoma) ; the whole being contained in the outermost, 

 capacious chamber (the body-chamber) of the shell, from which 

 the head can be protruded at will. The shell itself (fig. 248) 

 is involuted and many-chambered, the animal being contained 

 successively in each chamber, and retiring from it as its size 



