CEPHALOPODOUS MOLLUSCA. 253 



sion to bodies. In one genus, octopoda, which has not been 

 found in a fossil state, there are eight. In all the other 



FIG. 171. Ammonites Varians. Chalk. 



FIG. 172. Ammonites Mantelli. Clialk. 



genera there are ten of these acetabuliferous arms. The 

 shell, when it exists, is rarely external, and is mostly internal. 

 We only know one gemis, the argonauta (paper sailor), 

 which is protected by an external univalve shell. There is 

 no siphuncular apparatus, nor is the body of the animal 

 attached to it by muscles. We know a fossil species, 

 A. hians, from the sub-apennine of Piedmont. When the 

 shell is internal, it is horny, or testaceous, and is sometimes 

 provided with air-chambers and a siphon ; but the last 



