98 THE NAUTILUS. 



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

 compare the byssal 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- 

 uity, 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 operoulum. A detailed history of the growth of 

 the gasteropod operculum is lost in records of geological time. 

 Gray erred in considering the annular operculum to be the sim- 

 plest pattern. It is here advanced that its apparent simplicity 

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

 worn by Pteurotumana and Trocfms, 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 product will be 

 the paucispiral operculum. To maintain this kind of operculum 

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

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

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

 ing operculum, the first stage in evolution, or retrogression, pro- 

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

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

 t?m, Cerithhim or A'atica, in which other organs, such as the gill 

 plume, radula or nervous s) stem 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 

 eides and becomes concentric as in Vivipara, Vermttux, or Char- 

 on?'a. In another direction the increase is confined to one side 

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

 an unguiculate one like those of Stroinlus or Pyrene. 1 expect 

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



