VARIATION IN SIZE. 
H3 
the gigantic exainple.s of the same species found in Algeria, and our 
average Planorhis corneas are cpiite diminutive compared witli speci- 
mens of the same species from southern Europe. 
Inversely Balea perrersa would appear to find in North Britain and 
in elevated situations its most congenial habitats, the Scottish speci- 
mens being perhaps finer examples of the species than those from 
more southern and warmer localities. 
Granitic soils, peaty districts, or any foi’mation deficient of the 
calcic carbonate of which shells are mainly composed, are character- 
ized by shells not only of thin texture but dwarf size ; this diminu- 
tion of size is also an effect of altitude, crowding of individuals, and, 
according to iM. Locard, of darkness, or indeed of any other environ- 
ment unfavourable to the fullest development of the species ; the 
vicinity of the sea would also appear to have a dwarfing influence 
ui)on terrestrial and fluviatile mollmsks unless counter-balanced by 
other exceptionally favourable circumstances, and the same effect is 
})roduced by very deep waters, such localities also exercising a 
dwarfing effect upon the species inhabiting them. 
A diminutive size in freshwater shells may also bo a result of 
too great an abundance or overcrowding of individuals in the area 
occupied, as demonstrated by Semper, who has enunciated as a 
general princijde that huge bodies of water develop large shells, and 
ponds or other bodies of water much restricted in area produce small 
shells ; and this is asserted to be a general rule with fishes also, it 
being well known that the further one advances up-stream and the 
smaller the Trout become. 
The Colouring of the mollusca seems largely dependent upon the 
action of light, the more exposed surface of .spiral shells and the 
posterior end of burrowing Bivalves being usually more richly 
coloured or ornamented than the less exposed or buried portions, and 
colouring generally is probably of great biological importance ; it is 
also, as in other groups, most pronounced in brilliancy and variety 
in the warmer regions of the globe, and becomes gradually reduced in 
diversity and beauty as the poles are ai)proached. These known facts 
led Dr. Fischer to propose for the mollusca three zones of colouration 
corresponding with the thermal ones, viz. ; — 
PoLYCiiROMic, for the brilliantly coloured shells chietly inhabiting 
tropical countries. Examples : Licjuus virgineus (L.), West 
Indies, pi. ii., fig. 8, and Helix plcta Born, Cuba, pi. ii., fig. 9. 
