10 GASTEROPODA. 



Shell spiral ; adult and larva shell-bearing ; 

 larva with deciduous cephalic fins. 



Order IV. Gtmnobranchiata. — Gills exposed, 

 or contractile into a cavity on the surface of 

 the mantle. Adult shell-less; larva shell- 

 bearing, with deciduous cephalic fins. 



Order V. Pneumobranchiata. — Respiring free 

 air in a closed chamber lined with pulmonic 

 vessels. Adult and larva shell-bearing ; larva 

 shaped like the parent, without cephalic fins. 



The last order consists almost entirely of terres- 

 trial or fluviatile Mollusca, their organisation being 

 only adapted for respiring free air ; and there are a 

 few fluviatile species found in the first and second 

 order: the rest are all marine, and therefore ex- 

 cluded from our consideration at present. 



The operculum of Gasteropodous Mollusca is like 

 the shelly valve of those animals ; and the shelly 

 valve and the operculum together are homologous to 

 the two valves of a conchiferous mollusk. I am 

 therefore led to believe that the normal or typical 

 form of mollusca is, to be protected by two valves or 

 shells. If this theory be correct, the operculum 

 should aiford an important character for the distinc- 

 tion of families and genera. 



As this theory is not generally understood, I may 

 add that the operculum of these gasteropodous mol- 

 lusca, like the shelly valve of those animals and of 

 each of the valves of bivalves, — 



