492 



MOLLUSC A. 



FUSUS LONGISSIMUS. 



PEARL OXSTEE AXD COEAl,. 



VOLCTE. 



THORNY WOODCOCK. 



remaitis of the ammonite. The forms and colors of shells, as of all other objects, answer some 

 particular purpose, or obey some general law ; but besides this, there is much that seems specially 

 intended for our study, and 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 may be burned into lime; or in 

 shell-sand, for the use of formers. 



Not much is known respecting the individual duration of shell fish, though their length of life 

 must be very variable. Many of the aquatic species are annuals, fulfilling the cycle of their 

 existence in a single year; whole races are entombed in the 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 are born 

 in the summer time, in the warm shallows near the shore ; on the approach of winter they retire 



