THE MESOZOIC AGES. 223 



the Mesozoic, so that this may be called the special 

 age of Ammonites as well as of reptiles. 



Further, this time was likewise distinguished by 

 the introduction of true cuttle-fishes, the most re- 

 markable of which were those furnished with the 

 internal supports or "bones," known as Belemnites, 

 from a fancied resemblance to javelins or thunder- 

 bolts, a comparison at least as baseless as thai 

 often made in England of the Ammonites to fossil 

 snakes. The shell of the Belemnite is a most cu- 

 rious structure. Its usual general shape is a pointed 

 cylinder or elongated cone. At top it has a deej 

 cavity for the reception of certain of the viscera 

 of the animal. Below this is a conical series of 

 chambers, the Phragmacone ; and the lower half of 

 the shell is composed of a solid shelly mass or 

 guard, which, in its structure of radiating fibres 

 and concentric -layers, resembles a stalactite, or a 

 petrified piece of exogenous wood. This structure 

 was an internal shell or support like those of the 

 modern cuttle-fishes; but it is difficult to account 

 for its peculiarities, so much more complex than 

 in any existing species. The most rational suppo- 

 sition seems to be that it was intended to serve 

 the triple purpose of a support, a float, and a sinker. 

 Unlike the shell of a Nautilus, if thrown into the 

 water it would no doubt have sunk, and with the 

 pointed end first. Consequently, it was not a float 

 simply, but a float and sinker combined, and its 

 effect must have been to keep the animal at the 



