No. 6 (1921) COMMON MOLLUSCS OF SOUTH INDIA I6I 



The Date-shells {Lithodomus spp.) are long cylindrical 

 mussels, growing to a length of 3^ inches, which tunnel into 

 limestone rocks and corals, and sometimes into massive gastropod 

 shells such as the chank and the helmet-shell. Two of our common 

 Indian species are LitJiodoiiiiis a)itiUarum and L. stramineiis. Date- 

 shells bore only into shells, corals, and calcareous rock, the reason 

 being that the boring is effected by means of an acid secretion 

 poured out by glands in the mantle. The secretion decalcifying 

 the surface little by little enables the animal to scrape off and 

 remove the resultant debris and so permit of a fresh surface being 

 attacked. 



Of the A^r/z/V/r/c' only one conspicuous species is found in our 

 waters. This grows to a size of quite 5 inches in diameter ; it is 

 usually mistaken for a large pearl-oyster. In outline it is nearly the 

 same and the interior is lined with mother-of-pearl of good quality. 

 The hing,e is distinctive. Instead of a single oblong mass as in the 

 true pearl-ovster, the ligament is subdivided into numerous short 

 sections, each sunk in a separate little pit or fossette. The long 

 row of these fossettes renders Perna easily identified. It is rare and 

 I have seen only a few brought up from a depth of several fathoms 

 from rocky ground in the Gulf of Mannar. 



The large family of the AvicuUdae. is of great interest to Indian 

 students for in the waters of the Indian Ocean are found some of 

 the best known representatives ; they include the Pearl-oyster 

 {Mar.s;ai'itifn'a), the Wing-shells (Aviculids proper), the Hammer- 

 oysiei {Mulleins) and the curious Vulsella, 



The Indian and Ceylon PEARL-OYSTER {Margaritifcra vul- 

 garis), the niuthtlut chippi of Tamil pearl fishers, is amongst the 

 smallest of its kind ; it seldom exceeds 3^ inches in height (length 

 from hinge to ventral edge) and compares unfavourably in size with 

 the huge Gold-lip Pearl-oyster from Mergui and the South seas, a 

 species with shell large enough for a dinner plate. Our local 

 species makes up in abundance what it lacks in size ; at the 

 Ceylon pearl-fishery of 1905, over 80,000,000 of these shells were 

 fished within six weeks. The pearls produced are correspondingly 

 small, but in lustre, " skin " and colour, they far excel the larger 

 ones from the big species. The shell figured here shows the normal 

 form and size when grown under favourable conditions. The shell 

 is nearly straight along the hinge line, and is produced at each end 



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