No. I (1917) EDIBLE MOLLUSCS 1 5 



water by the employment of spat collectors ; experience shows that 

 ordinary country roofing tiles, arranged in low piles of several rows, 

 are the most suitable form to use. During our first experiments at 

 Pulicat, we arranged these collectors with a view to catch the spat 

 produced during the September spawning season ; experience shows 

 that it is preferable to aim at obtaining the necessary spat during 

 spring (.first half of April), as the parent oysters are in much better 

 health then ; the results obtained are altogether better, for opera- 

 tions can then be carried out with greater facility, the water level 

 in the parks being lower than it is in September. 



Fig. 3. — The Rock Oyster (Oslrea cnadlala). Natural size. 



The rock Oyster— Ostrea cucullata (Born). 



This oyster differs greatly from the backwater oyster. Its main 

 distinguishing characters are as follows : — 



Outline roughly oval ; the left valve extensively attached, deep 

 and cup-shaped, with a sacciform extension into the hollow beak 

 region of the hinge, which is moderately elongate in freely-grown 

 individuals ; the edges of this valve have a distinct tendency to 

 grow upwards. Externally the left valve is folded into deep ridges 

 passing radially outwards from the hinge and ending in a strongly 

 dentate edge which tightly interlocks with the edge of the 

 upper or right valve. The latter is flattened and opercular in form. 

 The muscle scar of both valves is usually dark purplish-black in 

 tint, rarely white. Very characteristic is a row of closely set 

 elongated denticulations seen a short distance inwards from the 

 margin on the inner surface of the upper valve ; these fit into a 



