IX 



SINISTRAL SHELLS 



249 



which intervenes like an operculum between the animal and the 

 substance to which it adheres. 



Sinlsti-al, or Left-handed Shells. — The vast majority of uni- 

 valve spiral shells are normally dextral^ i.e. when held spire 

 uppermost, with the aperture towards the 

 observer, 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 alto- 

 gether uncommon, and may be grouped under 

 four classes : — 



(1) Cases in which the genus is normally 

 sinistral; (2) cases in which the genus is 

 normally dextral^ but certain species are 

 normally sinistral; (3) cases in which the 

 shell is indifferently dextral or sinistral; 

 (4) cases in which both genus and species 

 are normally dextral., and a sinistral form is an abnormal 

 monstrosity. 



In all cases of sinistral monstrosity, and all in Avhich 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 well as the shell. 

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

 examined, and may therefore be assumed for the rest. The same 

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

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



Fig. 156. — Fulgur per- 

 versum L., Florida. x\i. 



A B C D E 



Fig. 157. — Illustration of the gradation of forms in Ampullarla between a dextral 

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



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

 but not always. In Spirialis^ Limacina^ Meladomus^ and Lanistes 

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

 anomaly has been most ingeniously explained by Simroth, Von 



