SHELL-FISH, OR MOLLUSCS 



743 



Ph,t. fy IT. S aV ill,.K,nt, F.Z.S.] [Milf,rd-,n.S. 



A SCORPION-SHELL 



Closely allied to the " Pelican's Foot " 



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. OYSTERS, 

 MUSSELS, COCKLES, SCALLOPS, 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 naturally in banks and beds of vast extent. In some species, such as the CEYLON 

 PEARL-OYSTER, 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 Ibs. 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 GIANT 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 by 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 

 exoosed to view when the 



Phot, bj W. Savillt-Knt, F.Z S. 



ROCK-OYSTERS shell-valves are partly open, 



This is the ordinary commercial oyster of the Australian shore* 



