_Sinistral, or Left-handed Shells._--The vast majority of univalve 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 altogether 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_. [Illustration: FIG. 156.--_Fulgur perversum_ L., Florida. × ½.] [Illustration: FIG. 157.--Illustration of the gradation of forms in _Ampullaria_ between a dextral (=A=) and an ultra-dextral species (=F=).] In all cases of sinistral monstrosity, and all in which 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, however, does not hold good in all cases for genera and species normally sinistral (sections 1 and 2). As a rule, the 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 Ihering, and Pelseneer. The shell, in all these cases, is not really sinistral, but _ultra-dextral_. Imagine the whorls of a dextral species capable of being flattened, as in a _Planorbis_, and continue the process, still pushing, as it were, the spire downwards until it occupies the place of the original umbilicus, becoming turned completely ‘inside out,’ and we have the whole explanation of these puzzling forms. The animal remains dextral, the shell has become sinistral. A convincing proof of the truth of this is furnished by the operculum. It is well known that the twist of the operculum varies with that of the shell; when the shell is dextral, the operculum is sinistral, with its nucleus near the columella, and _vice versâ_. In these ultra-dextral shells, however, where it is simply the method of the enrolment of the spire that comes in question, and not the formation of the whorls themselves, the operculum remains sinistral on the apparently sinistral shell.