270 MOLLUSCA. CEPHALOPODA 



the shell. From a study of this development of the 

 individual (ontogeny) some authors have attempted to 

 trace out the phylogeny of various types of Ammonoids 

 (see page 13). As in the case of the Brachiopoda (p. 167) 

 it has been found that some forms, which in the adult 

 state appear to be nearly identical, differ in their 

 development, suggesting that they have descended from 

 different ancestors. Similarly, as already mentioned, 

 changes may be seen in the form of the sutures when 

 traced from the beginning of the first whorl to later 

 parts of the shell; these changes are of importance in 

 the study of phylogeny, and seem to show that the 

 ammonites have descended from more than one group of 

 goniatites. 



Since the Ammonoids are extinct and their soft parts 

 unknown it is impossible to determine what number of 

 gills they possessed ; consequently their reference to the 

 Tetrabranchia cannot be definitely established. As, how- 

 ever, the shell was external and agrees closely in structure 

 with that of the Nautiloids, and the muscular impressions 

 in the last chamber are similar to those found in Nautilus, 

 we may regard the Ammonoids as closely allied to the 

 Nautiloidea; this view of their relationship receives 

 further support from the resemblance shown by the early 

 Ammonoids (in their relatively simple sutures, back- 

 wardly- directed septal necks, etc.) to the Nautiloids. The 

 protoconch of the Ammonoids, however, differs from that 

 of the Nautiloids and presents some resemblance to the 

 protoconch of Belemnites and some other Dibranchs 

 (page 282). 



Clymenia. Shell discoidal ; whorls numerous, more or less 

 flattened, all visible, but each partly embracing the preceding one ; 

 body-chamber long, generally occupying three-quarters of the last 



