102 
VARIATION IN BANDING. 
Fig. 213. — Limna-a pcrcgra (Mull.) 
X U> 
Scout Dam, Pcnlstone, Yorks., 
Collected by Mr. L. K. Adams, 1>. A., 
Showing the spiral linear mark- 
ings, probably due to injury to 
mantle-margin. 
The Spiral White Lines so often noticed, especially in our fresh- 
water shells, are not usnally what may be termed true colour bands, 
as those of typical Lunnaa 2wre(jra var. are said to be, but are 
probably the result of some injury to the 
glandular mantle-margin, which inter- 
feres with the secretion and deposition 
of the usual epidermic covering to the 
outer surface of the whorls, thus render- 
ing the calcareous strata of the shell 
more vividly perceptible. Mr. L. E. 
Adams has freipiently noticed this pecu- 
liarity amongst the Lhuntva jwrerjra 
inhabiting Scout Lam, near Penistone, Yorkshire, where trout are 
very numerous, and I consider these markings are })robably caused 
indirectly by the fish, which have 
injured or lacerated the mantle 
with their teeth when attempting 
to seize and feed upon the animal. 
More severe injuries of this cha- 
racter, from the same or other 
causes, would probably result in 
the entire thickness of the shell 
substance being more or less 
aflected, and in e.xtreme ea.ses the outer margin of the shell, where 
the new growth takes place, might even become cleft. 
In the Pelecypoda the same peculiarities are occasionally observable, 
the radiating dull white linear markings, when present, corresponding 
to the linear spiral markings of the Lunna klw, etc., and arc very 
proliably attributal)le in origin to analogous causes. 
.More severe injuries to the mantle, would, as in the Univalves, 
result in the com]ilete severance of the substance of the valve, but 
this severance is seldom so strikingly shown as in the specimen of 
A iKiiloidft cinjiti 'i ligured, which has the left valve cleft almost to the 
umbo, owing to the severe injury the corresponding mantle has sus- 
tained in early life, but the right valve shows only a very marked 
constriction to indicate the e.xtent of the injury done. 
'I'hese malformed shells should more pro2)erly have been treated 
upon in the succeeding cliaiher, but the desire to indicate the jn’o- 
Ijable connection of such forms with those distinguished by the 
Fig>. 211 21.5. — Lint/Ufa atirUtilaria L.), 
J>avos I.akc, Swit/criaml, 
CoIlccl^:tl by C'apt. W'. II. 'ruiTon, K.lh., 
Showing cleft ''licll >ubslancc probably tluc 
to severe laceration of niaiule-maigin. 
