THE AGE OF INVERTEHRATES OF THE SEA. 75 



attain to gigantic diinensions, beinjjj six feet or more in lengtli, 

 and nearly a foot in diameter. Thus the idea tiiat we should 

 naturally form from the study of the Nautilus, that it rejiresents 

 .1 type suited for much more varied and important ada[>tations 

 than those that we now see, is more than realised in those 

 Palaeozoic ages when these animals seem to have been the 

 lords of the seas. 



When we leave the Palaeozoic and enter the Mesozoic, though 

 the Nauliloid shells still abound, we find them suj)erscded, in 

 great part, by a nobler form, that of the Annnonitidai (iMgs. 71, 



Ya;. ■ji.—Ceratiles iiodosim (Sclilotli). Triussic. 



72). These arc remarkable for the ornate markings on the 

 surfaces of their shells, and for the beautifully waved edges of 

 the partitions (Fig. 72<7), which, by giving a much more com- 

 plete support to the sides of the shell, must have contributed 

 greatly to the union of lightness and strength so important to the 

 utility of the shell as a float. This type admits of r" the same 

 variety of straight, bent, and curled forms with the simpler 

 Nautiloid type, and some of the species are of great size, Am- 

 monites being known three feet or more in diameter. These 

 animals, unknown in the Palaeozoic, appear in numerous species 



