ENVIRONMENT OF THE OYSTER 93 



and 1-018 salinity. The water of a small creek, reduced by hot weather, 

 emptying between these two positions in a shaded ravine, was 16£° C. (61|° 

 F.) During the advance of warm weather the surface water is colder than 

 the air and deep water is colder and more saline than surface water. But this 

 may be modified by circumstances. As the air is more mobile than the 

 water, there may come a cold wave from a great distance and settle down 

 over warmer surface water, which may then become colder than the water 

 underneath and yet not sink, because of its less salinity. Sea water that 

 has been spread out on the surface of a bay may be brought up by the 

 tide and backed under colder river water. 



A bottom of some degree of hardness is required for oysters to rest 

 upon, otherwise they will sink and be smothered,. Rocks, stones, gravel, 

 shells, submerged logs, stakes, or other natural or artificial objects serve 

 very well, but these are generally lacking off shore, where the bottom is 

 most likely to consist of clay, sand, mud, sediment, and ooze. This is the 

 reason why on natural grounds there are so many places devoid of oysters. 

 The oyster overcomes this difficulty to a large extent through the dis- 

 persal of its larvae over wide areas; some of them may come in contact with 

 a chance object as a stone or a shell and become attached. This serves for 

 the fixation of many more in succeeding years, that in their turn furnish 

 fresh places for attachment until an oyster bed is formed. Dead shells 

 are as good as living oysters for this purpose, and if they break away from 

 one another they are all the more likely to roll apart and extend the base. 

 In time, a large area may be covered, the younger oysters building upon 

 the older until an oyster-bed, oyster-reef, oyster-bank, or so-called oyster- 

 rock of great breadth and several feet in thickness may result. 



The natural position for an oyster is to be fixed with its left valve to 

 the sub-stratum on which it rests, with the right valve uppermost. But they 

 cannot all find places on top; the surface of an oyster bed is very irregular, 

 with cuts, chasms, pinnacles, and ridges; and the free-swimming or creeping 

 larva may become fixed to any surface of a projecting shell with the result 

 that many are turned edge or end upwards. Spat that are separate 

 from one another when young, often impinge against one another 

 as growth proceeds. Many become abnormally lengthened, bent, twisted, 

 or otherwise distorted in shape by crowding. Sometimes the growth of 

 one presses upon and closes the shell of another to the extent that in time 

 it is prohibited from feeding. Such a rough, irregular, honeycombed mass 

 acts as a filter to the water that is moved by tides, currents, and waves, 

 effecting a deposition of silt, ooze, weeds, and other things carried in sus- 

 pension. This keeps settling into the interstices between the oysters until 

 the lower ones are buried and only the upper have free access to the food- 

 supplying water. Small scattered clusters occur that have originated in 

 a similar way to the beds. Single individuals, that have outgrown the 

 small objects to which they were attached, or that have been broken away 



