1302 BULLETIN OF THE BUREAU OF FISHERIES. 



but this arrangement, though an advance on previous work, is not capable of 

 comparative use, and any error in identification, not unHkely to occur with 

 persons not diatomists, would be misleading to future investigators. 



To overcome these difficulties I have for several years used the following 

 indirect method, which has given satisfactory results. The diatoms and other 

 food organisms are collected and counted, as before indicated, and are listed by 

 species, although their identification by their correct names is not essential. 

 Careful outline camera lucida drawings are made of the zonal and valvular 

 aspects of a number of specimens of each species, and their cubic contents are 

 calculated by geometric methods from planimeter measurements of the draw- 

 ings. The average of a number of such calculations will give the average rela- 

 tion of the volume to the product of length, breadth, and thickness of the 

 species. Using this relation and the average of a number of micrometer meas- 

 urements of the specimens themselves, a simple calculation will furnish an 

 approximately correct expression of the average volume of the species in the 

 region under investigation. If these volumes be employed as multipliers into 

 the numbers of the respective species, determined from the counts in the Rafter 

 cell, we have an approximately correct volumetric expression for the amount 

 of the food content of each specimen of water. As the most convenient unit of 

 measurement I have adopted Van Heurck's "c. d. m." (o.oi millimeter), the 

 unit of volume being the cube of this, "cu. c. d. m." (0.000,001 cubic milli- 

 meter) . The following is an illustration of the data required for each species : 



Synedra commutaia (Matagorda Bay) ; average length, 4.7 c. d. m.; breadth, 

 0.5 c. d. m.; thickness, 0.5 c. d. m.; volume = 0.6 (1 X b X t) =0.7 cu. c. d. m. 



This method sounds elaborate in its narration, but has not shown itself to be 

 cumbersome in practice, and, moreover, it appears to be the only method so 

 far proposed which gives data of real value. The results are directly compar- 

 able with those obtained in other waters or with those reached in the same 

 waters at different seasons. Five hundred or 600 determinations have been 

 made in the past two years, and, for reasons shown below, the procedure gen- 

 erally was found to require but little more labor than the older misleading and 

 less accurate method. 



In oyster investigations it is customary to take a large number of water 

 specimens at adjacent stations, and as the natiue of the food content of each 

 varies in quantity rather than in the character of the organisms, the measure- 

 ments of eight or ten species will apply to all water samples from the locality. 

 Only those organisms need be measured which examinations of the stomach 

 contents of the oysters show to be important as food. The counts have to be 



