The older zoölogists were inclined to ignore almost entirely the animal or "soft parts" of mollusks, and their classification of the phylum into "univalve," "bivalve," and "multivalve," based upon shell-characters alone, has since been proved to be wholly artificial. The terms "univalve" and "bivalve" are, of course, often and correctly used to-day to designate mollusks possessing a single or a double shell, such as the common garden-snail, which carries upon its back a single spiral shell, and, on the other hand, the oyster or clam, which is inclosed within two valves which fit closely together. But these terms are not /technically/ used, although they do indicate in a general way the more modern classification. Even among some comparatively recent conchologists a tendency to belittle the value of the "slimy creature" within the shell has led to some curious errors of grouping—mistakes which more thorough and scientific investigation has brought to light. In general, the higher classification into classes, orders, suborders, and families is exclusively based upon anatomical features, and to a large extent, also, is that of the genera; but species are always determined by their shell or conchological characteristics. When a certain familiarity with the forms and general appearance of shells is gained, a glance at the shell alone will almost always suffice to place it at once in its true generic position, for, along with modifications of the animal itself, corresponding modifications of the shell are quite certain to take place. Hence a fairly expert zoölogist should be able to determine with considerable accuracy from an empty and dead shell the anatomical features of the animal that at one time inhabited it. [pg308] 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