No. 6(1921) COMMON MOLLUSCS OF SOUTH INDIA 169 



apart by main force, the locking arrangement of the hinge is 

 cunningly devised. The ligament is a stout pad lodged centrally 

 in a deep triangular pit in the hinge area, with two very stout 

 interlocking teeth in each valve, guarding the ligament. The 

 adductor muscle is also specially strong. 



Order 2.— Eulamellibranchia. 



In these the gill filaments are united at regular intervals by 

 cross branches. Except in the oysters and a few others, two 

 adductor muscles are present. Many have the edges of the mantle 

 lobes united in one or two places posteriorly and these are often 

 prolonged into tubes or siphons, sometimes quite short, sometimes 

 extremely elongate. 



The true OYSTERS {Ostraeidae) are among the least typical of the 

 order and have much in common with some of the preceding order, 

 particularly with the Scallops and Spondyles. They all live a 

 sedentary life after the early free-swimming stage is past — the 

 description of the larval life of the pearl-oyster is practically 

 identical with that of all our South Indian oysters. Once they 

 settle to the bottom they turn over on to the left side (the converse 

 of the Spondyles and the Scallops) and cement the left valve to 

 whatever is handy — preferably a stone or another oyster, for they 

 are gregarious and form regular " beds." Sometimes they may even 

 attach to the roots of mangroves in backwaters. Their value to 

 mankind as food is greater than that of any other molkisc ; they are 

 susceptible to great improvement under cultivation, and the value 

 of their tonic qualities and easy digestibility when eaten raw is 

 difficult to over-estimate, particularly in the case of the old, and 

 those convalescent from illness. Three species are common in 

 South India, but one of these is local in habitat and does not form 

 beds, so that the number of our edible oysters is reduced to two, 

 the Backwater-oyster {Ostrea virgin iana) and the Rock-oyster 

 {0. cucullata). The former is the one specially valued and the one 

 v/hich lends itself readily to cultivation ; the second, while well 

 flavoured, is rather tougher as well as smaller than the other and 

 is seldom eaten except at one or two places on the Kanarese coast 

 where it is specially abundant. 



The Madras backwater oyster is extremely variable inform, 

 and passes under many names, scientific as well as local. Among 

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