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 faunas, 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 



