194 INVERTEBRATE ANIMALS. 



straight. In other nearly-allied forms the shell was bent or 

 even partially coiled up, but never so completely as in the 

 true Nautilus. Many of the Orthocerata were of small size, 

 but some of them were colossal, shells having been found of 

 six or seven feet in length, and as thick as the body of a man. 

 The Ammonites, with a number of allied forms of varied 

 shapes and beautiful structure, appear to have taken the place 

 of the Nautilidde, to a great extent, in the seas of the Second- 

 arj r period ; at which time, too, Dibranchiate Cephalopods first 

 made their appearance. The true Ammonites resembled the 

 Nautilus in having a many-chambered shell, which was coiled 

 up into a spiral, but the position of the siphuncle was differ- 

 ent, and the partitions or septa between the various chambers 

 of the shell were wonderfully folded and lobed instead of 

 being simply curved. The numerous beautiful shells allied to 

 the Ammonites cannot be even mentioned here ; but it is to 

 be remembered that they are almost all characteristic of the 

 Secondary period in geology, and that they are hardly known 

 as occurring in the older period (Palaeozoic epoch). 



