No, 6 (1921) COMMON MOLLUSCS OF SOUTH INDIA I67 



occurs frequently in immense numbers in the neighbourhood of the 

 pearl banks, clustering in myriads on seaweeds and zoophytes. 

 While quite young it is very difficult to distinguish from real pearl- 

 oyster spat ; only as growth proceeds do points of difference 

 emerge. 



One species of wing-shell is occasionally found on rough 

 bottom, of a size exceeding the common pearl-oyster. It is parti- 

 cularly common off the South Travancore coast. Its shell is brittle 

 and the nacre thin, so is of no commercial value. It may grow to 

 a length (height) of 7 inches. 



Closely related to the wing-shells is the HAMMER-OYSTER 

 {Malleus vulgaris), a black misshapen corrugated shell, shaped like 

 the letter T- The cross bar at the top represents the two enormously 

 developed ears, the upright, the high narrow body. When young 

 it has the general form and habits of an aviculid. With increas- 

 ing age it changes gradually into the monstrous form of the adult, 

 twisted, rough, and corrugated, and usually overgrown with crust- 

 ing growths of sponges, hydroids, polyzoa and ascidians. Like all 

 aviculids a portion of the inner lining of the valves is nacreous ; 

 but here is much less extensive than usual. 



Last of the family is the curious Vulsella rugosa, a deep oblong 

 shell without ears. It lives imbedded in sponges, not singly but 

 in numbers, so much so that an association appears to have 

 sprung up between the two, in the same way and for the same 

 reason that the gastropod Siliquaria forms a similar association 

 with another sponge. Vulsella is particularly common both north 

 and south of Rameswaram Island in depths of 3 to 5 fathoms. 

 The shell is roughened on the exterior and covered with a coarse 

 yellowish periostracum. No typical byssus is formed; some 

 authorities consider its differences from the true Aviculids sufficient 

 to entitle it to be put in a special family by itself — the Vulsellidae. 



The Scallops {Pectinidae) are not well represented in Indian 

 waters and are all small and insignificant unless we include the 

 magnificant smooth-shelled Aniussium, which by some is put in a 

 separate family, the Amussidae. 



The true scallops, such as the common Pecten senatorius, have a 

 deeply ribbed shell, usually round or oval in outline, with two well 

 developed and conspicuous " ears." In the young condition some 

 attach themselves to rocks and stones by means of a byssus ; 



