110 
MONSTROSITIES — SINISTRORSITY, ETC. 
Polseneor tliero arc no ;nlnlt (nistropod.^ winch are coiled oxoo-astri- 
cally, hnt ainonii.^t the Cejdialopods the i\d<?/t(’7»sfnrni.shes an example 
of this iirimitive arrangement. The original sinistrorsity or dextrorsity 
of the coiling is determined by the method in which this torsion is 
etfected : if twisted to the right the shell or coiling is dextral, if to the 
left, sinistral, hnt the direction of the torsion is naturally correlated 
with the arrangement nr disjiosition of the vital organs of the body. 
If we are correct in ap])lying the same terms to the Pelecypoda, 
and considering as the same jdienomenon the slight tendency to 
coiling exhibited by onr freshwater Bivalves and more nnmistahably 
by some marine forms as fm-urdia, etc., then we may regard all onr 
British species as Ihxogasti'ic nr IhiosoOYR.XTE forms, as the apices of 
their valves are directed anteriorly. None of onr freshwater species, 
and oidy a limited number of marine forms, as Tr'Kjmhi, etc., 
are really Bmlogastric or OinsTiioc.vR.VTE, that is with the nmhones 
directed towards the i)osterior margin of the shell. 
liev. Leonard .Tenyns in his monumental and classical work, “A 
Monograph on the Biitish 8])ecies of LV/c/a.s and has nn- 
fortiinately erroneously hgnred all the Pi^idid therein as Oiastho- 
gyrate or Lmlogastric, with the ajiices of the valves directed towards 
the shorter or jiosterior margin, instead of to the longer or anterior 
one, as they undoubtedly are ; this difference can he strikingly seen 
Kig. 2‘M>. — I^isitiium hoislaivanuiii (Shepp.), F tr,. 21^7. — Pisidiupi henslo^vnnum (Shepp.), 
adult, posterior end, X 12, yoiins, magnified (after Jenyns), 
Poiul, Cockerton, Darlington, Showing, lliougli erroneously, an exogastric 
Collected hy Mr. Charles Oldham, or o|)islh<\gyrate direction of the nmhones. 
Showing the endogaslric or prosogyrate 
direction of the um)>ones. 
Iiy reference to, and comparison of my figure of Phidl urn hrnnhunmum 
(tig. 2.‘h)), with tig. 2;)7, whicb is faithfully rc'produced from a. figure 
in Mr. .Tenyns’ Monogra])b. 
IIveERSTiiopiiv above; (rTp'x/jiy, turn). — Althongh sinistrally 
or dextrally coiled shells arc usually iuhabited by animals sinistrally 
or dextrally organized, as the case may he, yet exceptions are 
known in which a sinistrally organized animal is the tenant of an 
a])parently dextrally coiled .shell, and c/Vr ecr.s-d ; this curious feature 
which is exhibited by the genus Phinorhlf:, has always been a fruitful 
.source of di.scn.ssiou amongst conchologists, .some contending that 
