36 SHELL GALLERY. 
pearl-fishery of Ceylon the Rev. James Cordiner says that he saw 
the operation of sorting the pearls performed ; the produce of 
17,000 oysters weighed only J lb. and was contained in a vessel 
smaller than a common soup-plate. Out of that quantity there 
were not found two fine perfect pearls ; all of the largest were 
slightly deformed, rugged and uneven, but of the smaller sizes 
many were round and perfect. The chief qualities which regulate 
the value of pearls are size, roundness, and brilliancy of lustre. Of 
the smallest kind several may be bought for a shilling, whilst many 
thousand pounds have been given for a single fine pearl of sur- 
passing beauty. 
Other important pearl-fisheries besides that of Ceylon are carried 
on in the Persian Gulf, on the west coast of Central America, and 
especially North-west Australia, where diving-dresses are now 
employed in collecting the shells. 
The Chinese obtain pearls artificially from a species of fresh- 
water Mussel {Dipsas plicata). In order to do this they keep them 
in tanks and insert between the shell and the animal either small 
shot or small round pieces of mother-of-pearl, which soon receive 
regular coatings of nacre and assume the look of ordinary pearls. 
They also insert small metal images of Buddha, which also soon 
become covered with pearl and firmly cemented to the shell, the 
production being to the uninitiated a supernatural testimony to the 
truth of Buddhism. (A shell treated in this way is exhibited in one 
of the small cases (E) at the side of the room.) 
Cases 152- The Ostreidce, or Oysters, undoubtedly take the first rank among 
154. molluscs as regards usefulness to mankind as an article of food. 
They have no foot ; the mantle is entirely open, with double edges, 
each being bordered by a short fringe, and the labial palps are large 
and somewhat triangular. There are on each side a pair of simple 
gills, which appear closely striated ; the single adductor muscle is 
large and nearly central (see Fig. 30). The Oyster is, except in the 
very young state, entirely incapable of locomotion, and always 
attached by the deeper valve to other shells, rocks, or other sub- 
stances. The common British species is not full-grown until it is 
about five or seven years old. A series of different ages, from the 
"spat" to the adult form, is exhibited in Case 152. During the 
months of May, June, and July the eggs are discharged into 
the gills, where they remain until hatched ; and it is during 
this period that oysters are " out of season." In the American 
Oyster (0. virgin ica), on the contrary, the eggs are said to be 
