OYSTER BOTTOMS IN MATAGORDA BAY. 79 



merically about one-fifth, and in volume about one-eighth of the 

 available oyster food. Like the preceding species, it is taken up by 

 the oysters in relatively large numbers, and constitutes about 3.4 per 

 cent of their food. Average specimens measure in length 10 c. d. m., 

 breadth 0.T5 c. d. m., thickness 0.75 c. d. m., voTume=0.65 (lXbXt) = 

 3.5 cu. c. d. m. 



Melosira distans Kutzing (pi. xi, fig. 3) is a circular diatom fre- 

 quently aggregated by the circular faces to form filaments. It is 

 much more abundant below Dog Island, and is entirety absent in 

 Live Oak Bay and above Dressing Point. Between Mad Island and 

 Half Moon reefs it comprises numerically about 12 per cent, and in 

 volume over 25 per cent of the food contents of the water. It is taken 

 up by the oysters in about the same proportion relatively to its 

 abundance as C. lineatus, and in the lower bay constitutes about 12.3 

 per cent in volume of the stomach contents. Average specimens 

 measure about 3.3 c. d. m. in diameter and 2.3 c. d. m. in thickness, 

 volume=0.78 (d 2 Xt)=20 cu. c. d. m. 



Prorocentrum micans Ehrenberg (pi. xi, fig. 8), an infusorian, is 

 the last food organism of consequence and was found practically 

 everywhere in the bay. It was apparently lacking in section E, but 

 was in abundance on Dog Island Reef. It was about twice as 

 abundant in the lower bay as above this reef. It is less abundant 

 numerically than M. distans, and owing to its smaller size much less 

 important in quantity, but it is taken up by the oysters in such large 

 proportions that it constitutes about 4.3 per cent of their total food 

 contents. Average specimens measure in length 4.5 c. d. m., breadth 

 2.75 c. cl. m., thickness 1.4 c. d. m., volume=0.42 (lXbXt)=7 cu. 

 c. d. m. 



The other species of diatoms, constituting less than 7 per cent of 

 the oyster food; are as follows : 



Coscinodiscvs crassus Bailey (pi. xn, fig. 8). 



Navicula elliptica Kutzing (pi. xiii. figs. 1-6), length 4 c. d. m. ? 

 breadth 2.5 c. d. m., thickness 1.7 c. d. m.. volume =0.6 (lXbXt)=10 

 cubic c. d. m. 



N. arenaria Donkin, length 4.5 c. d. m., breadth 0.75 c. d. m., 

 thickness 1 c. d. m., volume=0.65 (lXbXt)=2^ cubic c. d. m. 



Amphora ovalis Kutzing, length 5 c. d. m., breadth 2.5 c. d. m. y 

 thickness 1 c. d. m., volume=0.4 (lXbXt)=5 cubic c. d. m. 



Pleurosigma fasciola W. Smith (pi. xiii. fig. 8), length 10 c. d. m., 

 breadth 1.25 c. d. m., thickness 0.5 c. d. m., volume=0.35 (lXbXt) = 

 2J cubic c. d. m. 



P. obscurum W. Smith, length 11 c. d. m., breadth 1.25 c. d. m=, 

 thickness 1 c. d. m., volume=0.75 (lXbXt)=10 cubic c. d. m. 



16354—07 m 6 



