16 



INTRODUCTION, 



Septa. 



The septa are simple in some species, as in the Nautilus, 

 fig. 13. In others they are undulated, having waved edges, as 

 in some species of Ammonites ; in others they are angulated, 

 as in Goniatites, fig. 480 in the plates ; and in the greater 

 number of instances, among the Ammonites, they are arbores- 

 cent, or branched, 





13. Section of Nautilus.— 14. Undulating Septa. — 15. Arborescent Septa, 



In the above section of a Nautilus, fig. 13, diminished in 

 size, showing the whorls and chambers (c), it will be seen that 

 the edges of the septa (s) are formed in one simple curve. In 

 fig. 14, the upper part of an Ammonite, the undulating line 

 will be seen ; and in fig. 15 a specimen is given of the arbo- 

 rescent septa. 



Sip la 



The Siphon is dorsal when placed near the outside of the 

 whorls ; central when near the middle ; and ventral when near 

 the inside of the whorl, or that part which leans against the 

 last volution, When it passes uninterruptedly from one 

 chamber to another, it is described as continuous, as in the 

 case of Spirula ; when, on the other hand, it only passes 

 through the septum a little distance, and opens into the 

 chamber, as in Nautilus, it is discontinuous. 



