G4 
VARIATION IN FORM. 
Fig. 137. — Vnio tumidus var. (Mont.), 
River Frome, Stapleton, Bri.stol, 
Collected by Miss F. M. Hele. 
Showing the compressed and somewhat curved pos- 
terior margin, and the slightly unequal \'alves. 
that the fine .specimen figured is from a lake near Noi’wich; possibly 
there may be some .special features in the locality, or of its occurrence 
there that would explain the seeming anomaly of its presence in still 
waters. In the variety ovidh of Unio tumidus the same peculiar 
compression of the pos- 
terior margin is exhibited, 
bnt the shell is somewhat 
inequivalve, the left valve 
being slightly concave on 
the posterior end, while 
the posterior end of the 
right valve is convex in 
even a greater degree. 
These and other resnlts of environment tend to give a general local 
likeness to allied species inhabiting the same river or lake — a fact 
which has often been noted and commented on. 
The (riimo.siTY which some species occasionally form by abruptly 
enlarging and afterwards contracting the 
diameter of the whorls, is very curions, and 
the suggestion has been made that a 
temporary accession of nourishing and 
palatable food during the growth period, 
would on account of the suddenly in- 
creasing hulk of tlie animal necessitate 
the unusual enlargement. Should this un- 
usual food-sup2)ly become exhausted, or not 
easily obtainable, the proportions of the 
animal would become gradually reduced to the normal size and the 
shell more or less quickly contracted in correlation with it. 
Mr. R. R. C. Stenrns ascribes analogous bulgings in the American 
PIdiiorIn'S merely to the periodicity of growth, owing to recurrent 
periods of hibernation, the termination of each growth period being 
marked by an expansion of the aperture, which may be consiilered 
analogous to a varix, and the repetition of these variceal enlarge- 
ments give the American species their special feature. The habit of 
forming these enlargements would be continued for a period, even 
though the species had migrated to districts where owing to more 
e(|uable conditions hibernation was nnnece.ssary, though in time 
they would revert to producing an even and regular growth. 
Fig. 138. — A Gibbously In- 
flated Univalve. 
Limmra aurkuiaria var. 
g-ihbosa Taylor, 
Pond, Moortown, near Leeds. 
