230 JOURNAL OF CONCHOLOGY, VOL. II, NO. 8, OCTOBER, I905. 



gastropods have dextral shells, it being the exception to find shells 

 built up with a left-handed twist. 



" But there is a subject to be investigated in connection with 

 sinistral molluscs more intricate than that which merely concerns 

 the reversed whorl of the external shell. The further question arises, 

 Is the internal animal, with its various organs, also reversed ? Is 

 there a strict correspondence between the shelly integument and the 

 mollusc, so that, if the outside spiral is sinistral, the soft inside parts 

 are also sinistrally placed ? Now the answer here is very varied. 



" First, in all cases where sinistrorsity occurs as an abnormal feature 

 {i.e., where the type is usually dextral, and a left-handed individual 

 is simply ' a freak '), this correspondence is maintained. The 

 internal organs in these instances are all ' reversed ' too, the viscera 

 being transposed in their relative positions. Second, in all examined 

 cases when the shell is as often dextral as sinistral, i.e., where it does 

 not seem that we can decide which is the really typical form as to the 

 direction of the whorl, the same rule obtains — the position of the 

 internal organs agrees with the shell. If the shell is sinistral, the 

 organs are sinistral ; if dextral, the soft parts are dextrally arranged 

 too. But in the case of genera which are normally sinistral, the 

 same law does not hold. Among molluscs also, whose genus is 

 normally dextral, but certain species of which are normally sinistral, 

 the strangest variations occur. A shell twisted to the left may have 

 the body of the animal twisted to the right, and vice-versa. In 

 Lii/uicina, A/eludo/nits, and Lanistes, for example, the calcareous 

 integument is sinistral, while the animal is dextral.^ What are we to 

 make of this ? How comes it that the internal animal, dwelling in a 

 house built on a left-handed spiral, has all its visceral organs arranged 

 as if they were designed to fit into a domicile reared on the principle 

 of a right-handed spiral ? The problem, which long seemed insoluble, 

 has recently been carefully investigated, and most ingeniously ex- 

 plained, through the researches of Simroth, of von Ihering, and 

 especially of Pelseneer. 



" Briefly, the theory which these scientists have advanced is that of 

 ' hyperstrophy' or 'over-turning,' and it amounts to this, that these 

 abnormalities and discordances between shell and animal are the 

 result, not of sinistrorsity, but of ultra-dextrality. Let us imagine a 

 jP// ri-a-shaped shell, with a spire normally sinistral, and an internal 

 animal also sinistral. Its spire is elongated, and it presents a 

 comparatively tapering appearance. Let us now suppose that this 

 elongated spire is so depressed and pushed into the body of the shell 

 that it projects very little above the level of the series of whorls. 

 That would be a second stage towards complete inversion of the 



I Rev. A. H. Cooke, "Molluscs and liracliiopods," Cambridge Nat. Hist., p. 249. 



