NO. 6 (1921) COMMON MOLLUSCS OF SOUTH INDIA 



175 



oysters are in much better health then; the results obtained are 

 altogether better, for operations can then be carried out with greater 

 facility, the water level in the parks being lower than it is in Sep- 

 tember and the salinity normal and not unduly reduced. 



The ROCK-OYSTER [Ostira aicuUata) differs greatly from the 

 backwater oyster. Its main distinguishing characters are as 

 follows : — ■ 



Outline roughly oval ; the left valve extensively attached, deep 

 and cupshaped, 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 



Fig. 42. ThQ^ock-oy^icr [Osirca I Hciilla la). Natural size. 



dentately lobate edge which tightly interlocks with the margin of 

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

 form. The muscle scar of both 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 

 corresponding series of furrows in the lower valve. Externally 

 the shell is tinted an opaque pinkish purple. Internally it is 

 white, margined with purple or black. The size is generally 

 smaller than that of the ordinary backwater or mud-oyster and 

 seldom exceeds 3 inches in length. 



The habitat of rock-oysters is in a narrow band between tide 

 marks; during ordinary spring tides, these oysters are entirely 



