76 OYSTER BOTTOMS IN MATAGORDA BAY. 



tified and counted, a second specimen is examined in the same man- 

 ner, and the sum of the twenty counts multiplied by 500 gives an 

 approximate to the total number of diatoms of each species in the 

 original liter of water. In former reports the writer has offered the 

 total number of diatoms as an index of the food value of the water, 

 but his experience in experimental work at Lynnhaven has shown 

 this method to be subject to grave error even as applied to a limited 

 region and to be very untrustworthy for purposes of comparison be- 

 tween different regions. As the species of diatoms vary widely in 

 size and fluctuate in relative abundance, it often happens that a mul- 

 titute of small ones give a fictitious value to a water specimen as com- 

 pared with another specimen containing a much smaller number of 

 a species of vastly greater volume. This is well illustrated in the 

 table on page 73. Comparing the water of Tiger Island channel with 

 that of Forked Bayou Reef, we find it to be but one-half as rich in 

 individual diatoms; but its food value, as computed by the method 

 hereafter explained, is found to be almost exactly one and two-thirds 

 as great, a disparity produced by the comparative abundance in the 

 former locality of C oscinodiscus lineatus, the largest diatom enter- 

 ing into the dietary of the oyster in Matagorda Bay, and in the latter 

 place of Synedra commutata^ the smallest species of importance. 

 Grave ° has recognized this and improves upon the previously em- 

 ployed method by disregarding in his report the smaller diatoms and 

 tabulating the larger, more important ones by species. His results 

 as published are interesting and valuable, but are difficult of com- 

 parison one with another and are still more difficult to bring into 

 relation with results obtained by the same method in other regions 

 producing diatoms of other species. Moreover, an error in the iden- 

 tification of the species, which may easily happen with persons not 

 specialists in the group, would entirely vitiate the results for pur- 

 poses of comparison by other workers. And finally, there is often 

 wide diversity in the sizes of individuals of the same species, some- 

 times small and again large ones predominating. 



In the present paper an attempt is made to estimate the actual vol- 

 ume of the oyster food in such manner as to make the results readily 

 available for comparison. To this end each species was carefully 

 measured in length and breadth and, wherever possible, in thickness. 

 In some cases the latter dimension was calculated proportionately from 

 published figures or estimated from the known thickness of a related 

 species. From these measurements and the figure of the diatom its 

 volume was calculated by ordinary methods, and this result was used 

 as a multiplier in arriving at the results shown in the tables on pages 

 72 and 73. It is not contended that this method is absolutely accurate, 



a Grave, Caswell. Investigations for the promotion of the oyster industry of 

 North Carolina, Report U. S. Fish Commission 1903, p. 247-351. 



