156 MADRAS FISHERIES BULLETIN VOL. XlV, 



hairs. The second species was well named Parallelopidum tortum 

 by some authors, for whilst the opposite sides are parallel none of 

 the angles are right angles and the shell appears as if it had been 

 twisted out of shape when in a plastic condition. Both of these 

 are fairly numerous in certain parts of Palk Bay and were shell-fish 

 more widely appreciated by Indians, they would find a place in 

 market supplies. The habitat of both is between the 4^ and 6 

 fathom lines on a bottom of dirty muddy sand. They do not form 

 continuous beds but occur scattered singly over large areas. Their 

 shells often give foothold to pearl oysters on the banks north-east 

 of Tondi. 



A small, elongated and very typical Ark-shell not unlike the 

 Twisted Ark-shell is found in crannies among massive corals, and 

 sometimes in the empty burrows of boring molluscs, especially 

 of the Date-shell. The foot is large and secretes a short and 

 massive horny byssus made up of thin plates of a characteristic 

 deep green colour. 



Belonging to the same family but very different in appearance 

 is the equivalve Pectunciiliis, a common shell in depths of a few 

 fathoms on the East Coast. In shape it is orbicular, somewhat com- 

 pressed and smooth, with central umbones. The hinge is curved 

 and so the line of the comb-like hinge teeth is somewhat arched 

 instead of being straight as in Area. The animal lives free and 

 unattached and so powerful is its crescent-shaped foot that 

 some species are able to leap to a height of several feet when 

 disturbed. P. taylori is a common species. 



The well-known MUSSELS [Mytilidae) form a large family. The 

 three common genera of these seas are Mytilus, Modiola and 

 Lithodoviiis. All have elongated shells with the hinge close to the 

 anterior end and without hinge teeth. In Mytilus, the umbones 

 are right at the anterior end; in Modiola they are some little 

 distance away. Both these live attached by byssal cables ; Litho- 

 domus is not attached by a byssus, but is equally sedentary as it 

 bores into limestones and corals, and does not leave its burrow 

 once it has formed it. 



Two species only of true Mussels grow to a considerable size 

 in the waters of this Presidency. One of these is the very hand- 

 some Green Mussel {Mytilus viridis), readily recognized because 



