60 
NOTE ON A VARIETY OF LIMN-EA 
STAGNALIS. 
J. W. TAYLOR. 
Leeds. 
THE interesting specimen of Zimnc@a stagnalis, found by Mr. 
Hutton (see p. 61), exhibits clearly and distinctly a character 
which is liable to occur in almost any of our Mollusca. Analogous 
specimens are occasionally found amongst the terrestrial species, 
but the peculiarity is much more frequently met with amongst 
the freshwater shells ; Zémncea peregra and Physa fontinalts 
being especially subject to this mode of ornamentation. White 
spiral banding of this kind is always adventitous, never com- 
mencing ab ovo, but in every case originating during the free 
life of the animal. It is probably an effect arising from injury 
to, or laceration of the outer margin of the mantle by fish or 
some other predatory creature, by which the secretory glandules 
are injured or destroyed, and as the particular glands secreting 
the outer or epidermal layer are the most externally placed, they 
are the most liable to injury and destruction. 
Injury to the secretory cellules results in a deficient secretion 
of the protective outer epidermis, which in the Limnzeidz gives 
the colour to the shell, so that its absence or unusual delicacy 
and thinness exposes the white calcareous stratum beneath, and 
enables it to become strikingly perceptible and in strong con- 
trast to the horny-brown colour of the adjacent uninjured shell 
surface, each injured gland or group of glands giving rise toa 
slender or broader white or whitish line, which revolves spirally 
around the whorls in strict correlation with the direction of the 
coiling, while the rate of increase in its breadth is in correspon- 
dence with the general enlargement of the shell itself. 
If the laceration of the marginal glands is not so severe as 
to totally destroy, but merely tears the glandular fringe, the 
injury may in process of time become healed, and the white 
revolving lines on the shell, due to the injury, will be gradually 
obliterated by the overgrowth of epidermis of normal density 
and appearance. 
If, however, the injury to the pallial margin be more serious, 
then the more deeply seated lime cells may also be destroyed, 
in such case, not merely is the outer epidermal layer involved ; 
but the substance of the shell itself may show its effects by the 
shell wall being distinctly cleft. 
Naturalist, 
