3«4 



MOLLUSCA. 



very striking. The fresh-water snail Planorbis has an 

 unchambered shell spirally coiled in one plane ; but in this 

 and in similar Gasteropods the foot is turned towards the 

 internal curve of the coil, while that of Nautilus is directed 

 externally. 



There are only about half a dozen living species of 

 Nautilus, 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 

 believing that these belong to the Dibranchiate section of 

 Cephalopods. 



The following table states the chief points of distinction 

 between Nautilus and the other series of Cephalopods : — 



Cephalopoda, 



