{8 MANUAL OF 1HE MOLLUSCA. 
and they are the ornaments of barbarous tribes. The Friendly- 
Islander wears the orange-cowry as a mark of chieftainship 
(Stutchbury), and the New Zealander polishes the elenchus into 
an ornament more briliant than the ‘‘ pearl ear-drop” of 
classical or modern times. (Clarke.) One of the most beautiful 
substances in nature is the shell-opal, formed of the remains of 
the ammonite. The forms and colours of shells (as of all other 
natural 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 farmers.* 
Not much is known respecting the individual duration of the 
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, and lakes, and seas; thus, in the Wealden clay we 
find layer aboye layer of small river-snails, alternating with 
thin strata of sediment, the index of immeasurably distant 
years. Dredgers find that whilst 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 larger size 
than any that we find alive, because they obtain 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 
because they have induced voyagers to collect. Mere shell-collecting, however, is no 
more scientific than pigeon-fancying, or the study of old china. For educational pur- 
poses the best shells are the types of genera, or species which illustrate particular 
points of structure ; and, fortunately ‘or students, the prices have been mich diminished 
of late years. A Carinaria, once ‘ worth 100 guineas” (Sowerby), is now worth Is. 
only ; a wentle-trap which fetched 40 guineas in 1701 (Rumphius) was worti only 
20 guineas in 1753, and may now be hed for5s. The Conus gloria-maris has fetched 
£50 more than once, and Oyprea umbilicate has been sold for £30. 
* Shell-sand is only beneficial on peaty soils, or heavy clay land. It sometimes 
hardens into limestone, as on the coast of Devon; and at Guadaloupe, where it con- 
tains littoral shells and human skeletons of recent date. 
