OYSTER BOTTOMS IN MATAGORDA BAY. 77 



but it gives good approximate values readily available for compar- 

 ison with other investigations made by the same method and will in 

 a measure place the study of oyster food upon a volumetric basis. 

 It has the advantage also of placing less importance upon the abso- 

 lute identification of the diatoms, for if the measurements be accu- 

 rately made and the figures carefulh T drawn the volume can be calcu- 

 lated without reference to the exact names of the species. 



The unit of measurement adopted in this report is that employed 

 by Van Heurck in his Treatise on the Diatomacea?, the one-hundredth 

 part of 1 millimeter (0.01 mm. =0.0003937 inch), referred to as a 

 " c. d. m." (centieme decimeter). The unit of volume, which is re- 

 garded as presumably the unit of food value, is of course the cube of 

 this, or one-millionth of 1 cubic millimeter (0.000001 c. mm.). It 

 follows from this that when, as in section A of the table on page 73, 

 the food value of the water is said to be 219,342, it is meant that in 

 absolute measurement 1 liter of water contains diatoms of an aggre- 

 gate volume of about one- fourth of 1 cubic millimeter. 



In order to make the results of greater value for comparison and to 

 render them susceptible to recasting to accord with such improvements 

 as may be introduced into the method above outlined, there should be 

 given for each species, or at least for all of the important ones, the 

 following data : Xame, or the name of closely allied species ; outline 

 of its figure; average length, breadth, and thickness, preferably in 

 c. d. m. ; its calculated volume ; the number per liter of water, as 

 determined by the Rafter method. Ordinarily it will be unnecessary 

 to furnish these facts for all of the species, as it will be found that in 

 any region from 4 to 8 organisms constitute the great preponderance 

 of oyster food and the other species found are negligible for all 

 practical purposes. In Matagorda Bay there were found in the 

 stomachs of oysters about 25 species of diatoms and 1 infusorian, 

 but over 98 per cent of the food in bulk was contributed by 

 8 organisms, Coscinodiscus lineatus, C. excentricus, Navicula didyma, 

 N. elliptica, Synedra commutata, Synedra sp., Melosira distans, 

 and Prorocentrurn micans. The figure and the actual numbers of 

 each species in each locality will be found in the accompanying tables 

 and illustrative plates, and all the other data in the following notes on 

 the several species. The identifications were verified by Dr. Alfred 

 Mann, and with one or two minor exceptions are authoritative. The 

 measurements given are the average dimensions of a number of indi- 

 viduals of each species. 



DESCRIPTION OF ORGANISMS CONSTITUTING FOOD OF OYSTERS IN 



MATAGORDA BAY. 



Coscinodiscus lineatus Ehrenberg (pi. xn, figs. 1-3) is a large 

 circular diatom, wdiich on account of its bulk and general distribution 



