46 OKHAMANDAL MARINE ZOOLOGY REPORT 



in robust health in the open sea and at a depth of several fathoms. Thus, I have seen 

 quantities thrown up on the open beach at Nilaveli, some seven miles north of 

 Trincomalee, and, again, during a cruise of the steam trawler Violet, during the past 

 summer (1907) along the Coromandel coast, numerous specimens were trawled in the 

 open sea from a muddy bottom in seven- fathoms, at a distance from seven to nine 

 miles eastward of Negapatam ; I have also several dead valves trawled from the 

 Periya Par, one of the Mannar pearl-oyster banks, situated twelve miles off the west 

 coast of Ceylon. More recently I have found living individuals cast up during stormy 

 weatTier along the coast adjacent to Madras. 



The shells of those individuals that live in shallow bays and creeks are perfectly 

 transparent for the greater part of the first year of life ; the general anatomy of the 

 animal can be seen, even to the beating of the heart. At this stage the valves are 

 clear as the finest mica flakes, which they resemble closely, even to the readiness with 

 which they may be split into further laminae. As they become older the valves 

 assume a more massive appearance, and generally turn white and translucent in place 

 of being clear and transparent. 



The very young, up to the time they reach a diameter of two inches, not 

 infrequently exhibit ray bands of pale transparent pink diverging from the hinge, 

 and broadening as they approach the free edge of the shell. A few are almost entirely 

 suffused with this pale pink tinge, but the great majority exhibit no colour even in the 

 youngest stages, and all shallow- water shells become colourless and sub-translucent after 

 one year and a half. 



A marked distinction between the shallow-water (brackish water) forms and those 

 I have seen from the open sea is that in the latter a well-marked reddish-brown radial 

 banding of the shell appears to be normal. It was characteristic of the great majority 

 of the trawled individuals, and was persistent in the largest specimens examined (twelve 

 centimetres diameter), whereas, as already stated, red colouring is transient, and 

 comparatively uncommon in those found in muddy bays. The tinge among the latter 

 is also much fainter, lacking the intensity and depth that is a noteworthy characteristic 

 of those from several fathoms in the open sea. 



The bottom favoured by Placuna placenta is a fairly stiff or pasty greyish-black 

 mud. On this the shells generally lie prone upon their convex left valves, the hinge 

 region sometimes slightly sunk in the mud, which may lightly cover the dorsal 

 third of the shell. Native divers also aver that occasionally they are sunk more deeply, 

 and that they may even be found, though this is rare, planted vertically after the 

 manner of Pinna ; if so, this is an abnormal position, and one which, I believe, must 

 quickly entail death. 



Few conspicuous animals are associated with Placuna 'placenta. Both flora and 

 fauna are scarce ; few organisms have adapted themselves to life in such environment. 

 In Tampalakam Bay the only other animal of importance and abundance is the peculiar 



