40 
FORMS OF SHELL IN lUVALVES. 
longer side and in Pi.i/diniii at the shorter end of the shell ; this 
striking distinction was overlooked by Dr. 
Jeffreys, who, in his “ British Conchology,” 
erroneously described the posterior side of 
the shell of Pis/Jiinii as that most produced. 
AVhen the nnibo 
is situate about tbe 
centre of a syinnietri- 
that the outline and 
and posterior sides 
correspond, the shell is called Eohilateral; fk-,. oo.— a Sub-equiia- 
^ . ’ tenil liivalve. 
some si)ecies of the genus SiiInci'/uiii have , ■'>■//,«■/■/«/« rAwoAr(Lcacii), 
^ O J X 1.',. 
almost e(inilatora.l shells. m 
-* Lull, by Mr. W. D.CnckjI'.O.S. 
I' u;. iK 3 . — An .\iUeriorly pro- 
tluced Inequilateral lb\'alve. 
I ''isid'nun a mnicimi ( M ill 1 . ), 
right valve, X 2. 
Canal, Ambergate, 
Collected by Rev. H. Milne,'^. 
cally formed shell, so 
area of the anterior 
Reniform shells are often caused by some injury to the mantle 
margin, causing a deticieut secretion at the injured part, and if this 
be along the ventral surface a more or less reniform or kidney-shaped 
shell results, the amount of siituation depending upon the nature 
and amount of the injury sustained hy the animal. The Phid'min 
sbvtatam Bourg. is founded on a specimen of Phid'mm cinemun 
Fk;. 97- — A Reniforni liivalve. 
Unio inargariti/cr \ZiX. sinuata Lain., right valve, 
Loch .\\ve, Argyleshire, collected by Mr. A. Somerville, II. Sc., F.L.S. 
affected in this way. Lhdio nHirgaritif'r var. Pnmdta is an apparent 
e.xample of this peculiarity of shape occurring normally, though Dr. 
dray ascribes the form even in this case to the e.xcoriatiou of the 
umbones. Mr. Madison has specimens of Anodontd c.yg)ie(i which, 
owing to some injury, show this peculiarity in a very extreme form, 
the valves being (piite cleft nearly to the umbones. 
