MOLLUSCA. CEPHALOPODA 269 



(p. 236). A similar structure, but consisting of chitin, 

 and with the two plates united, is found in the body- 

 chamber of some ammonites. 



In a few Ammonoids the shell is either a straight 

 cone (e.g. Bactrites) or coiled into a helicoid spiral (e.g. 

 Turrilites), but in the great majority of the genera all the 

 whorls are in a plane spiral, and in such the form of the 

 shell depends mainly on whether the later whorls grow 

 round the earlier, or are simply in contact with them ; in 

 some genera the last whorl partly (fig. 119) or completely 

 (fig. 112) conceals all the previous ones, but in others 

 (fig. 114) the whole of the whorls are visible, and then 

 the umbilicus — which is present on both sides of the shell 

 — is very large. When the diameter of the whorl from 

 side to side (i.e. the thickness) is greater than the diameter 

 from the internal to the external margin (i.e. the height) 

 then the umbilicus becomes deep, and if in such cases the 

 later whorls embrace the earlier, then the umbilicus will 

 be both deep and narrow. 



The surface of the shell may be smooth or orna- 

 mented with striae, ribs, tubercles, or spines. In some 

 ammonites (fig. 117) the external margin of the shell is 

 provided with a ridge or keel, and in these forms the ribs 

 of the two sides are not continuous. The keel may be 

 smooth or toothed. In some genera there is either a 

 groove or a flattened margin in place of the keel. The 

 aperture of the shell is frequently produced into lobes at 

 the sides, or into a pointed projection at the external 

 margin (figs. 117, 118). 



The character of the ornamentation changes at different 

 periods in the life of the individual ; these changes, which 

 occurred during growth from the protoconch up to the 

 adult, can be traced out by examining the early whorls of 



