r.o 
A'ARIATION IN CHARACTER OF LIP. 
stances would occur later than in the gibbons variety, and only just 
prior to the final completion of the shell. Although the majority of 
known specimens showing an expanded lip 
are from tranquil waters, yet this peculiarity 
has been assumed to be also the result of 
the animal living in turbulent waters, com- 
pelling it to cling closely to stones, etc., to 
avoid being detached and washed away. 
An analogous cause suggested hy several 
observers is that it may be a conseiiuence of 
the animal being compelled to reside mainly 
upon and crawl over fiat and hard surfaces 
during the growth period, as upon tank 
sides in caidivity, or upon the hardened mud of ponds which have 
become partially dried up. 
Expanded or reflected lip growth has however often been observed 
to occur during the autumn when the 
reproductive season has terminated and 
the cessation of this drain upon the 
system enables the food consumed to 
contribute solely to maintain and increase 
the bulk of the animal and may thus 
necessitate an expansion of the shell ; but 
the enlargement may also occur at the 
normal growth period, if suitable food 
be abnormally plentiful and ethological 
conditions fiivourable to such increase. 
Fig. U2. — An Univalve with Ex- 
pancleti lip. 
Linnuca sta^^nnlis (L.), 
Ash ton -under- Eyne, 
Collected hy the late Mr. Lister 
Peace. 
Fig. in. — An Univalve with 
Reflected lip. 
L. f>eregrii v. lahlosa JefTr. X 2, 
( ireenhc.'ul P.irk, Huddersfield, 
Collectetl hy Mr. J. Whitwham. 
'I’be Armature of the aperture or mouth of 
Fig. 113. — Pupa scccilc Drap. X 1, Fig. 141. — Pupa sccaie 
Mailing Hill, near Lewes, var. edentula Taylor X 4, 
Collected hy Rev'. S. S. Pearce, M. A., Ingleton, Yorks., 
Showing the typical form and a perfectly edentulous variety of 
a normally strongly dentate or plicate species. 
other countries develop a varying number of 
the shell is a feature 
which is usually the 
attribute of the per- 
fectly ad\dt state. 
Some species, as 
Vertigo mlniitis- 
simii, may exist in 
our own country in 
a perfectly edentu- 
lous form, which in 
denticles contracting 
