CEPHALOPODA 



605 



whorls, but particularly by the patterns of the suture line^ that is, the 

 junction line of the septum and tlie outer shell (V\^. 413). These 

 patterns reach the greatest complexity. A great deal of interest at- 

 taches to the fact that in these characters the earlier formed chambers 

 of an ammonoid individual usually differ from those of the adult shell 

 (h'igs. 413, 414, 415). 'J'here may, in fact, be several changes in the 

 life of an individual and the succession of such changes has been re- 

 corded as evidence for tracing the descent of particular ammonoids. 

 The most striking manifestation of the phenomenon is afforded by 



Of 



ni 





<^i 



Es Li S 





Li Lz I 



a « 



i '«• 4^5- 



Fig. 414. 



Fi;^. 414. Baculites chicoensis Chalk. After Perrin Smith. 



Fig. 415. Suture lines of Baculites to show the variation in development at 

 different af^es. a, first, h, second and c, sixth suture lines of B. rhkoensis \ 

 d, adult septum of B. capensis ; E. external lobe ; Es. external saddle ; /. internal 

 lobe ; //J , L^ , first and second lateral lobes ; .Sj , first lateral saddle ; .S'2 , internal 

 (dorsal) saddle, a-c after Perrin Smith, d 'dftcr Spath. 



ammonoid stocks, particularly in the Cretaceous, in which the ap- 

 proach of extinction is heralded by " uncoiling" in various stages. In 

 Scaphttes the shell is coiled in youth but later straightens out and 

 finally hooks back. In Baculites (Fig. 414) only the very earliest 

 chambers forjn a coiled shell; nearly the whole of the shell is straight. 

 But the suture lines, though tending to become simplified, show the 

 type of the family from which the uncoiled form is derived, and it is 

 possible to show quite definitely that such genera as ** Scaphttes" and 

 ''Baculites" are not natural but polyphyletic; both scaphoid and 

 baculoid forms occur in different lines of descent. 



