98 THE NAUTILUS. 



But Houpsay in 1884 showed that it would be more correct to 

 compare the bj-ssal gland of the bivalve foot to the mucous gland 

 of the gasteropod foot than to its operculum. 



In molluscan ancestry the operculum should be of high antiq- 

 uit}', for not only was it possessed by ammonites, but it exists 

 in the tubicolous worms. Perhaps the valves of a chiton indi- 

 cate metameric repetition of what in the gasteropod developed 

 as shell and operculum. A detailed history of the growth of 

 the gasteropod opercuhim is lost in records of geological time. 

 Gray erred in considering the annular open-ulum to be the sim- 

 plest X)attern. It is here advanced that its apparent simjdicity 

 is that of degeneracy. The round, multispiral, horny operculum 

 worn by Pleurotomnria and Trochvs, though actually far ad- 

 vanced, is yet the mosL primitive operculum found among recent 

 gasteropoda. 



From a gland that secretes new matter on its growing edge, 

 this multispiral operculum must be carried backwards rapidly. 

 By "rapidly" is meant that it describes many revolutions in a 

 lifetime. Let rotation be slowed down, and the j)roduct will be 

 the paucispiral operculum. To maintain this kind of operculum 

 with on area equivalent to that of the mullispiral type, each 

 spiral has to be greatly broadened. The shape has now clianged 

 from round to oblong. So starting from a round, quickly-turn- 

 ing operculum, the first stage in evolution, or retrogression, })ro- 

 duces an oblong, slowly-turning operculum. It is suggested 

 that this type of operculum is associated with forms like Littor- 

 ina, Cerithivm or Aatica, in which other organs, such as the gill 

 plume, radula or nervous system have not attained extremes of 

 differentiation. 



Again, let rotation continue to slow down, till the operculum 

 ceases to revolve upon its muscle. The product, an operculum 

 without axial movement, may now proceed along different lines 

 of development. In one direction it adds fresh growth upon all 

 sides and becomes concentric as in Viviparn, Verw<tiu'<, or C/mr- 

 onia. In another directicm the increase is confined to one side 

 and may result in a lamellar operculum like thnt of Thais, or 

 an unguiculate one like those of Sirovilms or Fyrene. I expect 

 that when traced to early stages, both the lamellar and ungui- 



