492 
MOLLUSCA. 
FUSUS LONGISSIMUS. PBAEL OlSTJia AND COBAL. VOLUTE. THORNY WOODCOCK. 
remains of tlie ammonite. The forms and colors of shells, as of all other ohjects, answer some 
particular purpose, or obey some general law; but besides this, there is muclithat seems specially 
intended for our study, aud calculated to call forth enlightened admiration. Thus the tints of 
many shells are concealed during life by a dull external coat, and the pearly halls of the nautilus 
are seen by no other eyes than ours. Or, descending to mere utility, how many tracts of coast 
are destitute of limestone, but abound in shell-banks, which maybe burned into hme; or in 
shell-sand, for the use of farmers. 
Not much is known respecting the individual duration of shell-fish, though their length of life 
must bo very variable. Many of the aquatic species are annuals, fulfilling the cycle of their 
existence in a single year ; whole races are entombed in tke wintry tide of mud that grows 
from year to year in the beds of rivers, lakes, and seas • thus, in the Wealden clay of England 
layer above layer of small river-snails are found, alternately with thin strata of sediment — the 
index of immeasurably distant years. Dredgers find that while the adults of some shell-fish 
can be taken at all seasons, others can be obtained late in the autumn or winter only, those 
caught in spring and summer being young, or half-grown ; and it is a common remark that dead 
shells, of some species, can be obtained of a much larger size than any that we find alive, because 
they attain their full growth at a season when our researches are suspended. Some species 
require part of two years for their full development ; the young of the doris and eolis arc born 
in the summer time, in the warm shallows near the shore ; on the approach of winter they retire 
