CEPHALOPODA 



529 



These are differentiated by the shape and sculpture of the shell 

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

 junction line of the septum and the outer shell (Fig. 388). These 

 patterns reach the greatest complexity. A great deal of interest attaches 

 to the fact that in these characters the earlier formed chambers of an 

 ammonoid individual usually differ from those of the adult shell (Figs. 

 388, 389, 390). There may, in fact, be several changes in the life of an 

 individual and the succession of such changes has been recorded as 

 evidence for tracing the descent of particular ammonoids. The most 

 striking manifestation of the phenomenon is afforded by ammonoid 



Fig. 390. 



Fig. 389. 

 Fig. 389. Baculites chicoensis Chalk. After Perrin Smith. 

 Fig. 390. Suture lines of Baculites to show the variation in development at 

 different ages, a, first, b, second and c, sixth suture lines of B. chicoensis; 

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

 ternal lobe; Lj, Lg, first and second lateral lobes; S^, first lateral saddle; 

 So, internal (dorsal) saddle, a-c after Perrin Smith, d after Spath. 



Stocks, particularly in the Cretaceous, in which the approach of 

 extinction is heralded by "uncoiling" in various stages. In Scaphites 

 the shell is coiled in youth but later straightens out and finally hooks 

 back. In Baculites (Fig. 389) only the very earliest chambers form 

 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 " Scaphites " and " Baculites " 

 are not natural but polyphyletic ; both scaphoid and baculoid forms 

 occur in different lines of descent. 



Bi 34 



