THE CEPHALOPODA 183 



a siphuncle along the ventral margin and a forward 

 prolongation of the dorsal region, the pro-ostracum 

 (Fig. 53, a, b, b') and (2) a solid guard, more or less 

 cigar-shaped, with a conical hollow (alveolus) at the 

 blunt end for the reception of the apex of the phragmo- 

 cone. The guard is composed of fibrous calcite arranged 

 radially ; it is more often found preserved than the 

 phragmocone, and is so resistant to wear and tear that 

 belemnite guards are often found as derived fossils in 

 gravels. In a few cases, sufficient traces of the body of 

 the belemnite have been found to show that the skeleton 

 was entirely internal, the animal growing out of all pro- 

 portion to the growth of the shell, so that an ever smaller 

 portion of the body remained within the body-chamber, 

 and the mantle gradually enveloped the whole shell. 

 Like its relative the modern cuttle-fish, it possessed an 

 ink-sac and a number of arms around the mouth, though 

 these were furnished with horny hooks not known in the 

 cuttle-fish, and there seem to have been only six of them 

 instead of the eight or ten of modern dibranchiates 

 (Fig. 53, a). 



In some of the earliest (Upper Triassic and Lower 

 Jurassic forms) the phragmocone extends nearly to the 

 end of the guard (Fig. 53, c), and in these cases it is 

 probable that the former was still an external shell in 

 early life and O 7 nly later became enveloped in the mantle, 

 which then began to secrete a guard around it. (This 

 makes the name " guard " inappropriate, since it was 

 formed when the phragmocone least needed protection). 

 But in most later forms the alveolus only extends a 



