THE SLUG AND ITS ALLIES 



173 



family of Ammonites, of which two thousand species are 

 known, ranging from the Silurian up to the Cretaceous 

 formations. 



FIG. 164. Nautilus pompilius, the Pearly Nautilus. Median section, c, outer- 

 most chamber whose bottom is formed by the septum, se ; s, siphon travers- 

 ing all compartments ; e,eye; h, hood; in, part of mantle, reflected over 

 the shell; I, lobes enclosing tentacles (<) ; si, incomplete funnel; im^ shell 

 muscle ; n, gland secreting the capsules for eggs. After Owen. 



