16 



ner, and 45 to the outer surfaces. Here only 64 per cent, were 

 attached to the inner surfaces. This percentage was somewhat 

 lower than in the case of the other reefs and may find its explana- 

 tion in the fact that these shells were almost "mudded up," so that 

 there was less than the normal difference between their inner and 

 outer surfaces. The count made in September in the same locality 

 showed on 100 shells only 120 living young oysters of which 61 

 per cent, were on the inner surfaces. Thus the total number of 

 spat counted was smaller than earlier in the season. Even had it 

 been equal to the former number, the second count would have 

 shown, as compared with results from reefs i, 2, and 3, which 

 were clean com.pared with those in the mouth of St. John's Bayou, 

 the deleterious effects of mud, the constant and overwhelming 

 presence of which not only reduces the surface available for the 

 attachment of new spat, but kills that which has been unfortunate 

 enough to settle within range of its deadly influence. 



Two observations on the number of spat caught on reef No. 4, 

 the "Light-house reef," (see map) were made in August and 

 September. In the former count 398 young oysters were found 

 on 100 shells of which 300 or 75 per cent, were on the inner sur- 

 faces and 98 on the outer. The count in September showed only 

 349 young oysters on icx) shells, 249 of which were on the inner 

 surfaces. This discrepancy between the two counts is due partly 

 to the fact that a number of young oysters died during tne inter- 

 vening period, and partly also to the fact that one end of reef No. 

 4 runs almost up to high water mark whereas the other end extends 

 outward into the deepest part of the river. Thus it is probable 

 that the physical conditions between these two extremes may ac- 

 count for the fact that more oysters were found at the first count 

 than at the second as the former was taken farther from shore 

 than the latter. 



RATE OF GROWTH. 



The rate of growth of the young oysters was determined by 

 comparing the lengths of a chosen number at two different times. 

 For this purpose 20 young oysters still attached to shells were 

 selected, carefully measured, and marked to facilitate recognition. 

 The distance from the tip of the umbo of the upper valve to the 

 middle of the anterior edge of that valve was taken arbitrarily as 

 the length. On August 27th the oysters selected had the lengths 

 given in Table II and on September 27th one month later the pro- 

 portions given in Table III. 



