SHELL-FISH, OR MOLLUSCS 
345 
Leaf-gilled group. Though not so numerous 
in species as the last, it outrivals it in the 
enormous abundance in which the individuals 
of many varieties are produced. Ov.STERS, 
Mu.ssels, Cockle.s, Sc.allops, and other 
allied forms occur in closely associated 
colonies, constituting natural “ beds ” or 
“ banks,” which may be of vast extent and, 
in at any rate the case of oysters, several 
feet in thickness. From a commercial and 
economic standpoint this group is un- 
doubtedly of the highest importance to the 
human race. Not only do its members, as 
instanced by the foregoing forms, contribute 
largely to the world’s commissariat, they also 
yield the much-prized material known as “ mother-of-pearl ” and the purest and most aesthetically 
beautiful gems — orient pearls. Pearls and mother-of-pearl are the products of two groups 
of shell-fish, respectively known as Pearl-OYSTERS and Pearl-MUSSELS. There are a consider- 
able number of species, mainly denizens of tropical seas, which, like ordinary oysters and 
mussels, occur naturall)' in banks and beds of vast extent. In some species, such as the CEYLON 
Pearl-OYS'I'ER, the shell is small, and the mother-of-pearl substance, or “ nacre,” as it is technically 
termed, so thin as to be of relatively little value. Hence the fishery for this species is 
conducted almost exclusively for the sake of the pearls, which are fairly numerous and 
frequently of the finest quality. From the tropical Australian seas pearl-shells of the largest 
size, which produce the thickest and most valuable mother-of-pearl, are obtained. Pearls 
of the best quality are more rarely found in this description of shell, and its fishery is 
prosecuted primarily on account of the substantial substance and magnificent quality of its 
nacre. A single pair of shells of this species will attain in its adult state to a weight of 
from 12 to 1 8 lbs. The fishery for this pearl-shell has, however, been prosecuted so relent- 
lessly that bivalves of such matured age and weight are now of rare occurrence, and obtained 
only from almost inaccessibly deep waters. Unless, in point of fact, systematic methods of 
conservation and cultivation are resorted to on an extensive scale and on lines corresponding 
fundamentally with those successfully followed in the culture of ordinary commercial oysters, 
there would seem to be an imminent risk of the valuable Australian pearl-shell fisheries 
becoming depleted to more or less complete exhaustion. 
The tropical Australian seas, and notably those which wash the Great Barrier Reef, are 
famous for the production of the largest of living bivalve molluscs. These are represented by 
the Gl\NT Clams, which, dwelling among the coral-growths, are left exposed to view for brief 
periods during abnormally low spring tides. A photograph of a colony of these monster. 
bivalves, taken b\' the writer 
amidst this mollusc’s char- 
acteristic surroundings, is re- 
produced on page 741. The 
example in the foreground 
measured no less than 4 
feet in diameter and weighed 
several hundred-weights. In 
many clams the living tissues, 
or mantle-borders, that are 
exposed to view when the 
shell-valves are partly open, 
This is the ordinary commercial oyster of the Australian shores are brilliantly tinted. 
PAofo hy W. SaviUc-Kent, F.Z S. 
ROCK-OYSTERS 
