BELEMNITES. 



351 



hing along the smaller curve in the same way as in the Ammo- 

 nites (Fig. 649). This structure, as we shall see, is very much 

 like that prevailing amongst the species of the following order, 

 and which will be explained hereafter ; but the shell, instead of 

 being external, is completely enclosed within the posterior part 

 of the body of the animal. 



965. With this order we are also to 

 associate an extremely interesting fami- 

 ly, that of the Belemnites, whose remains 

 abound in several of the older rocks, es- 

 pecially the Lias and Oolite. The shell 

 Consists of a conical-chambered portion, 

 implanted (as it were) into a corre- 

 sponding hollow of a long solid sheath, 

 tapering to a point at its lower extrem- 

 ity (Fig. 647). The conical chambered 

 portion extends considerably beyond the 

 hollow of the stony sheath ; but the 

 border of the latter seems to have been 

 prolonged forwards in a horny condition, 

 so as still to envelope the chambered 

 cone. In the last or largest chamber of 

 the cone, distinct remains of an ink-bag 

 have been frequently met with ; from 

 which it has been argued, that, notwith- 

 standing the strong resemblance of the 

 shell to that of many genera allied to the 

 true Nautilus and belonging to the Te- 

 trabranchiate group, the animal must 

 have been Dibranchiate, and must have 

 included the shell, together with its mas- 

 sive sheath, in the same manner as the Cuttle-fish included the 

 pounce-bone. This argument was founded upon the fact, that 

 the Nautilus the only Tetrabranchiate Cephalopod now living 

 possesses no ink-bag ; its power of withdrawing the body 

 completely into its shell, rendering such a means of protection 

 unnecessary : and its justice has been made evident by the dis- 



FlG. 647 BELEMNITES; 



a, large extremity, showing the 

 insertion of; the chambered 

 cone into the sheath. 



