On Retiersed Shells. 221 



§ 12. On Reversed Shells. 



The non-symmetrical shells are often what are usually called 

 reversed or sinistral shells, that is, belonging to animals which have 

 their organs of respiration placed on the right instead of the left 

 side in the CtenobranchouSj and the aperture of the respiratory 

 cavity on the left instead of the right, in the Pneumonobranchous 

 Mollusca, so that they are somewhat analogous to left-handed 

 persons of the human race; but they are much more seriously 

 affected, for the whole of their viscera are equally reversed, as 

 was pointed out by Lister, who wrote a very interesting paper on 

 this subject. 



In the shell of all these animals, the whorles of the spire, instead 

 of turning and descending from the left to the right, turn the con- 

 trary way, that is from the right to the left, so that the shell if 

 placed in its natural position, that is with its apex toward the 

 observer, with its mouth downwards, will have the latter placed 

 on the left side of the axis. 



This kind of structure, which is mostly an accidental variety, is 

 sometimes natural to a whole group of shells, when their whorles 

 revolve, and their form is much more regular than when it is only 

 a monstrosity, as is the case with the Physce, Planorbes, Clan- 

 silice, Sfc. There are other species and groups where it is not 

 uncommon to find some specimens, or species, in one way, and 

 the others in the other, as in some Pyrulce, Achatina;, Ampul- 

 lariw, and Helices, whilst in other groups this is of very rare oc- 

 currence. This peculiarity has been generally considered as con- 

 fined to the spiral univalves. But in my description of the shells in 

 the Appendix to Capt. Parry's first Voyage, I first described it as 

 found in the bivalves, which may be considered as formed of a 

 dextral and sinistral univalve shell united together by their 

 mouths. Since that time I have observed the same fact in several 

 other species. There are two specimens of a new Lucina,* in the 

 Tankerville collection, which, as pointed out by my friend Mr. 



* I call this new shell Lucina ChildrintB ; it is distinguished from all the other 

 LuciniB by being unequivalved, and approaching nearly in form to the Cy- 

 thcrea exuleta; there is also another specimen in the British Museum. 



