io6 RAMBLES AND REVERIES. 



so as to form a strong hood with which to close the 

 shell. There is a nervous system, consisting mainly 

 of two masses or ganglia, protected partially by a 

 cartilaginous plate which we may regard as a kind 

 of rudimentary skull. It is clear then that the 

 creature is by no means a lowly organised one ; 

 indeed, it is placed at the very top of the Mollusca, 

 which are only a short step behind the Vertebrates. 

 The function of the chambers of the shell is 

 probably that of enabling the animal to alter its 

 specific gravity, but what are the precise uses of the 

 siphuncle does not seem to be understood. 



FIG. 21. Foliation of Ammonite chambers. 



The chief differences between a Nautilus and an 

 Ammonite have to do with the walls that divide 

 the chambers. In the case of the former the divi- 

 sions or septa are simple, and where they join the 

 shell the lines are plain. But the septa of Ammonites 

 have frilled edges of many patterns. These patterns 

 form very beautiful traceries on the exterior shells 

 of Ammonites. A representation of one of these 

 diversified patterns is given (Fig. 21). 



It is worthy of remark, too, that the Nautilus 

 family made its appearance in the waters of our 



