MOLLUSKS 307 
to choose this field of natural history for special study. But the 
true student of zodlogy does not confine his investigations in this 
phylum to considerations of the shell only, for the protective 
armor constitutes only a portion, although an important portion, 
of the anatomy of the mollusk. On the same principle, it would 
hardly suffice to examine only the skeleton of vertebrate animals 
in order to become acquainted with the fishes or mammals. Ina 
sense, the testaceous covering of a mollusk may be likened to an 
outer skeleton. 
The older zodlogists 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 plaasifcation; 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 in- 
vestigation has brought to light. In general, the higher classi- 
fication 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 ap- 
pearance 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 modi- 
fications of the shell are quite certain to take place. Hence a 
fairly expert zodlogist should be able to determine with con- 
siderable accuracy from an empty and dead shell the anatomical 
features of the animal that at one time inhabited it. 
