SINISTRAL SHELLS 



249 



which intervenes like an operculum l)etween the animal and 11 le 

 substance to which it adheres. 



Sinistral, or Left-handed Shells. — The vast majority of mii- 

 valve spiral shells are normally dextral, i.e. wdien held spire 

 uppermost, with the aperture towards the 

 oliserver, the aperture is to the right of the 

 axis of the spire. If we imagine such a 

 shell to be a spiral staircase, as we ascended 

 it we should always have the axis of the 

 spire to our left. 



Sinistral or ' reversed ' forms are not 

 altogether uncommon, and may be grouped 

 under four classes : — 



(1) Cases in whicli the. genus is normcdly 

 sinistral ; (2) cases in which the r/enus is 

 normrdhj dextral, but certetin S2)eeies are 

 normcdly sinistral ; (3) cases in which the 

 shell is indifferently dextral or sinistrcd ; 

 (4) eases in which loth r/enus and species 

 are normcdly dextraJ, and a sinistral form is an ahnormcd 

 monstrosity. 



In all cases of sinistral monstrosity, and all in wliicli a sinistral 

 and dextral form are interchangeable (sections 3 and 4 above), 

 the position of the apertures of the internal organs appears to be 

 relatively affected, i.e. the body is sinistral, as w^ell as the shell. 

 This has been proved to be the case in all specimens liitlierto 

 examined, and may tlierefore be assumed for the rest. Tlie same 

 uniformity, however, does not hold good in all cases for genera 

 and species normally sinistral (sections 1 and 2). As a rule, the 



Fio. 156.- — Fulgur perrer- 

 sum L., Florida, x h. 



Fig. 157.- 



-Tllustration of tlic gradation of forms in Amiinllaria between a dextral 

 (A) and an ultra-dextral sjjecies (F). 



anal and genital apertures are, in these instances also, to the left, 

 but not always. In S-piricdis, Limacina, Meladomus, and Lanistes 

 the shell is sinistral, but the animal is dextral. This apparent 

 anomaly has been most ingeniously explained by Simroth, A'on 



