72 THE ORIGIN OF MAN 



may be drawn out into a sharp point, the rostrum. 

 In nautilids the mouth of the shell is never closed 

 by an operculum, but in the goniatids and am- 

 monids it probably was closed in most forms by 

 a covering when the animal was at rest. But 

 these differences are, however, not so important 

 as is the nature of the septa. In the nautilids 

 these partitions in the chambered shell are simple 

 and more or less concave, while in the ammonids 

 they are simply only in the central part, and each 

 septum becomes more and more fluted or wavy 

 toward its junction with the outer shell. Late in 

 the Silurian, the deep-shelled nautilids gave rise 

 to small and narrow-shelled goniatids with spar- 

 ingly lobed septa. In most goniatids of the De- 

 vonian, the suture line of the lobes and saddles 

 terminates sharply, but in the Mississippian the 

 majority of the species not only have more lobes 

 and saddles, but these are nearly all rounded. 

 Thus we see a gradual change in the narrow- 

 shelled cephalopods, beginning as true nautilids, 



