308 MARINE INVERTEBRATES 
One of the main reasons for the special interest that seems to 
have attached to the study of conchology is derived from the 
fact that the durability of shells has caused their almost perfect 
preservation as fossils from the very earliest periods of geological 
time, thus furnishing the key to the solution of many problems of 
evolution. Upon this page of the earth’s history the letters are 
sharp and clear, and geologists possessing a knowledge of recent 
forms of mollusks as well as of their geographical distribution 
have been enabled to read some wonderful stories of the cosmic 
history of islands and continents. 
GEOGRAPHICAL DISTRIBUTION 
Mollusks are found in every part of the world. The arctic 
seas possess their own characteristic faune, the more temperate 
waters of Europe and of America contain their own peculiar 
genera and species, and the warm waters of the tropic seas furnish 
the conditions favorable to the life of an immense number of 
characteristic forms. Again, there are the littoral species, that 
live only between tide-marks and are therefore exposed to the 
air for a number of hours each day; some of these live just about 
high-tide mark and have become almost terrestrial in their habits, 
while others must be sought at the point of lowest tide, where 
for only a short time each day they are deprived of their natural 
element. Then there are the shallow-water forms, which never 
appear above low-tide mark save when a neap tide surprises them ; 
their range in depth extends to about the hundred-fathom line, 
more or less. Beyond this depth to several hundred fathoms 
other characteristic forms appear, and from the more profound 
depths of mid-ocean the dredge has brought to light a host of 
curious and interesting species. 
It is the temperature of the water rather than the depth that 
appears to influence the distribution of marine mollusks. Thus, 
certain species whose natural home is in the shallow waters of the 
Arctic Ocean have been taken in very deep waters off the southern 
coast of the United States, the temperature conditions in both 
stations being substantially the same. This fact, however, must 
not be too freely accepted as establishing a principle. Some 
