108 
MONSTROlSITIEy— i^INISTRORSITY, ETC. 
Tlie SiNiSTRORSiTY and Dextrorsity of the inolluscaii shell is a 
very intricate and perplexing subject, which in some genera has given 
rise to considerable discussion. The great majority of Gastropoda 
have undoubtedly dextral shells, that is with the whorls turning spirally 
from left to right, with the heart on the left side of the animal, and 
the external apertures of the various oi’gans on the right. The 
sinistra! or left-handed coiling is thus the exception or monstro- 
sity, althongh some species and even genera are normally sinistral, 
and possess an opposite arrangement of their internal organs to 
dextral species, and the dextrally coiled shell becomes the exception 
or monstrosity. Reversed monstrosities likewise conform to this 
disposition of the vi.scera, a dextral monstrosity of a normally sinistral 
■shell having its organs and orilices disposed as in those mollusks 
which are normally dextral. This reversal of the direction of the 
coiling, though liable to occur in all spiral shells, is of much greater 
rarity in some species than in others — for, of Iltdiv nttniiJuta, our 
commonest species of lldlv, only one recent sinistral specimen is 
known, which was found by Rev. II. W. Lett, at Loughbrickland, in the 
north of Ireland, and through his kindness is now in my collection. 
All mollu.sks w’ith spiral shells are liable to this reversal in the 
direction of their convolution, and bivalves and even slugs are affected 
in an analogous way. In bivalves tlie right or left valves, as the case 
may be, ac(piire the characters which normally distingui.sh the other, 
but this reversal when it does occur is not very noticeable except 
perhai)s in .some of the ineipiivalve .species. In the naked genera, the 
occurrence of this state is outwardly shown by the transference of the 
respiratory and other orifices to the side opposite to that on which 
they are normally placed. 
Locality would ajipear to have .some inllnence in inducing the 
reversed coiling of shells, some hjcalities being well-known h^r the 
regnlar recurrence of these monstrosities, while in other districts they 
are scarcely known to occur. Cailliaud records the neighbourhocal of 
Rochelle as noted for the fre(iuent occurrence of sinistral //c//.c«.sy^e/vs'((, 
and l)r. Gwyn .leffreys states that he himself saw a colony of that 
monstrosity in the garden of M. d’Orbigny in that city. 
A sinistral race of llell.r /(cwo/'n/hs, almost analogous to that formerly 
existent of Fiisns aiitiqiiiis, would appear to have at one time lived in 
county Donegal, as the very numerous subfossil .shells picked out of 
the immense sand-hills about Rundoran abundantly testify. 
