62 
VARIATION IN FORM. 
Land shells of elongate form, are often somewhat arboreal in habit 
or at least more inclined to ascend to elevated positions, than the flat 
spired forms, which woidd appear to he in many cases more geo- 
philons, or ground-loving in habit; onr most depressed species. Helix 
obvoluta is particularly noticeable for this peculiarity, naturally 
there are exceptions to this rule, which however indicates in a general 
way the tendencies of the various forms. 
Fluviatile species tend to develop a lengthened shell under tlie 
influence of a steady and rapid current, and short spired forms, 
of otherwise elongate shells, more or 
less characterize species inhabiting 
lakes and other large bodies of water, 
Liii/iKiu iK'vetjra var. bunietti and 
Lliuinvd stagimlis var. bodamica may 
he instanced as e.xemplifications of 
this rule, and, as recorded at page 59, 
Herr Ilazay has produced the long- 
spired variety oi Li mncra 'peregm at 
will from the ova of the globose form 
by merely rearing them in a rapid current of water. 
In the Pelecypods the shape may he modified by the greater or 
lesser convexity of the valves, which may he considered analogous to 
the conoid and depressed varieties in the Univalves. In those species 
Tic. 13o. — L'nio iionidus IMiil., 
(Iroby Pool, Leicestershire, collected by Mr. H. Ik. (^iiilter. 
Sliowing n regularly iiHlated burrowing llivalve, usually characteristic of still waters 
(this specimen shows the pecidiarity of three tlistitjcl and successive margins to each valve). 
which do not regularly imbed a portion of their shells in the bed of 
the stream or pond, the tumidity or compression of the shell is some- 
what uniform over the whole shell, hut in the species composing the 
genera J and Unio, which usually live partly buried in the 
Fic.. 131. — A Short spired variety 
of an elongated Univalve. 
Lininua stdi^tialis var. bodamJcii Miller, 
Koden See, 
Collected by Herr Schenk. 
