THE CEPHALOPODA 293 



the form of the aperture and the position of the " hyponomous " 

 sinus, which corresponds to the funnel (Fig. 261, i.s). In some 

 Xautiloidea, e.g. the dextral or sinistral Trochoceras, and in sundry 

 Ammonoidea, e.g. the sinistral Turrilites and Cochloceras and the 

 dextral Bostrychoceras, the coil may be produced into a helicoidal 

 or turriculated spire. And in other cases again the last whorls 

 of shell, whether it be discoidal or helicoidal, may be partly un- 

 coiled, as may be seen, for example, in Lituites, which is largely 

 uncoiled, or in Ophidioceras, in which only a small extent of the 

 shell is uncoiled. Finally, the shell may become secondarily 

 rectilinear in the adult, as in Baculites, among the Ammonoidea. 

 In Spirilla the shell is coiled in one plane, but it is endogastric, 

 that is to say, coiled in the opposite direction to that of Nautilus 

 (Figs. 268 and 270), and it is largely internal. In certain fossil 

 Dibranchia the multilocular shell, whether it be straight or partially 

 coiled, has become internal (Belemnitidae, Spirulirostra} and forms 

 the phragmocone (Fig. 262, C). In such cases it is surrounded by a 

 calcareous secretion of the reflected portion of the mantle, which 

 is not homologous with the shell of other Molluscs, and forms the" 

 pointed rostrum or guard at the end opposite to the head (Fig. 262) 

 and the cephalic plate or pro-ostracum at the anterior or dorsal end. 

 Thus there is, in the shells of these Cephalopoda, an element which 

 is not represented in the shells of other Mollusca. 



In the living Dibranchia, with the exception of Spirula, the 

 phragmocone and the rostrum of this internal shell have become 

 very rudimentary. In Sepia, for example, the shell is composed of 

 parallel layers united together by short pillars of calcareous sub- 

 stance, and has a stratified and alveolar structure : at its posterior 

 end a little hollow marks the position of the phragmocone, and a 

 short pointed external projection represents the rostrum, the bulk 

 of the shell being formed by the anterior pro-ostracum, on which the 

 retractor muscles of the cephalopedal mass are inserted. In the 

 Oigopsida the guard is no longer calcified, and the shell has the form 

 of a chitinous flume or gladius, but in Ommatostreplies there is a 

 small posterior conical cavity representing the remains of the 

 phragmocone. 



In the Loliginidae and Sepiolidae the shell is similarly repre- 

 sented by a chitinous gladius (Fig. 263), but in these families it is 

 so much reduced that it only occupies the anterior portion of the 

 body. In Idiosepius this shell is nearly obsolete, and it is absent 

 altogether in certain Sepiolidae and some allied forms such as 

 Stoloteuthis, Inioteuthis, Sepioloidea, and Sepiadarium. Finally, in the 

 Octopoda there is no longer a true shell, but only some simple 

 chitinous rudiments, on which the retractor muscles of the head 

 and funnel are inserted ; these may be paired, as in the case of the 

 lateral stylets of Octopus ; or unpaired, as in the case of Cirrhoteuthis. 



