208 



HOMOLOGIES OT UNITALTE SHELL AXD OPEECULUM. 



^^ metapodium ;" and as the " mesopodimn,^'' so called, derives its 

 name from its intermediate position between the left-mantle 

 pedicle and the '•'■ propodmm^'' it is so far incorrectly named ; I 

 therefore substitute for it the term " iasipodiumP 



Common tabular Plan of the Organisation of Gasteropoda and 

 Conchifera, showing also their points of difference. 



With the above principles before us, the doctrine that the uni- 

 valve shell, irrespective of the operculum, is the equivalent of 

 both valves of the Conchifer united together, is quite untenable ; 

 nor does the prima facie comparison of Leptori squamosum with 

 TrocJius (such having been chosen by Mr. Woodward in support 

 of this view) establish its truth one whit. It must be remembered 

 also that it devolves upon the upholders of this improbable piece of 

 transcendentalism to dispose of the operculum, which in many cases 

 assumes almost as much importance as the shell itself. Many 

 conchologists regard the operculum merely as a glutinous or 

 horny secretion, sometimes the nidus of calcareous deposit, adapted 

 to the form of the mouth of the shell as a fm'ther protection to 

 the retracted animal ; but beyond this, in tlieir estimation, it 



