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Differences between the specie
It can apppear difficult to discern the
differences between the two specie of flat periwinkle, but in reality
it is quite easy:
Littorina fabalis is smaller; usually has a yellow body; a lemon-yellow
or light brown shell; the shell spire is usually absent; and is common
on bladder
and toothed
wrack.
Littorina obtusata is larger; usually has a black body; the shell
is usually olive-green, dark brown or light yellow; a large whorl above
the shell aperture is quite common; and is commonly found on knotted
wrack.
Both specie can be discerned by the appearance
of their genitalia that is concealed by the shell. It is
Littorina fabalis that is portrayed on the first
page, in the upper left hand corner, aswell as on the
next page.
Littorina obtusata is pictured above.
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