430 PHYLUM MOLLUSCA. 
usually unsymmetrical, and the foot (ventral) is turned 
towards the internal curve of the coil, while in Nautilus 
the dorsal part of the animal is towards the internal 
surface of the chamber. 
Fic. 233.—The Pearly Nautilus (Mazte/us pompilius). 
—After Owen. 
The shell is represented in section, but the animal is not dissected. 
Part of the mantle has been removed. c¢., Last or body 
chamber, separated by a septum (se.) from the compartment 
behind ; s., the siphuncle traversing all the compartments ; 7z., 
the portion of the mantle which is reflected over the shell; 4., 
the hood ; ¢., the eye with its opening to the exterior; 7, the 
lobes which bear the sheathed tentacles (#.); sz., the incomplete 
sipaons mit, the shell muscle ; ., the position of the nidamental 
gland. 
There are only about half a dozen living species of 
NVautilus, but there are many hundred fossils of this and 
allied genera. This list is usually swelled by the addition 
of the extinct Ammonites, but there are some reasons for 
