78 OYSTER BOTTOMS IN MATAGORDA BAY. 



is the most import ant food organism of the bay. It is found in 

 practically equal profusion both above and below Dog Island, and 

 an examination of the stomach contents of the oysters from the prin- 

 cipal reefs shows that it constitutes about 63 per cent of the food. 

 It lives on or near the bottom, and is suspended in the water most 

 abundantly in the presence of strong currents or energetic wave 

 action. Average specimens measure in diameter 5 c. d. m., thickness 

 1.75 c. d. m., volume=0.78 (d 2 Xt)=35 cu. c. d. m. 



Coscinodiscus excentricus Ehrenberg (pi. xn, figs. 4-7) is a small 

 circular diatom practically uniformly distributed, excepting in Live 

 Oak Bay and the waters above Dressing Point, where it is deficient. 

 In its vertical distribution it resembles the preceding species, and its 

 numerical abundance is about one-half. Proportionally to its .abun- 

 dance in the water it is consumed in larger numbers, but owing to its 

 smaller bulk it constitutes but about 10 per cent of the food found in 

 the oysters' stomachs. Measurements of average specimens show the 

 diameter 2.25 c. d. in., thickness 1.7 c. d. m., volume=0.7 (d 2 Xt)=6 

 cu. c. d. m. 



Navicula didyma Ehrenberg (pi. xiii, figs. 7-11) is an 8-shaped dia- 

 tom, found in much smaller numbers than either of the foregoing 

 and not so universal^ distributed. It was altogether lacking in 

 four sections, and is considerably more abundant and constant below 

 than above Dog Island Reef. It constitutes about 1.8 per cent of the 

 food of the oysters in the lower part of the bay. Average specimens 

 measure in length 4 c. d. m., breadth 2.25 c. d. m., thickness 1.8 



c. <l. m., volume=0.7 (lXbXt)=ll cu. c. d. m. 



Synedra commutata Grunow (pi. xi, fig. 7) is a very small and 

 active boat-shaped diatom which is important by reason of its ex- 

 traordinary abundance in the lower bay, especially in the vicinity of 

 Forked Bayou Reef, where numerically it constitutes over 80 per cent 

 of the total diatom content of the water. It was found in every 

 section and at almost every station, but varies sharply in its numbers 

 on the two sides of Dog Island Reef, the average per liter in section 

 E being 11,650, and in section F but 2,000, while the average in the 

 lower bay is over six times that of the upper bay sections. It fur- 

 nishes in bulk about 9 per cent of the food of all oysters in the lower 

 bay, though on Forked Bayou Reef this average rises to upward of 

 30 per cent. Average specimens are in length 4.7 c. d. ms., breadth 

 0.5 c. d. m„ thickness 0.5 c. d. m., volume=0.6 (lXbXt)=0.7 cu. c. 



d. m. 



Synedra species? (pi. xi, fig. 5) is an active diatom much longer 

 i ha n the preceding species. It is universally distributed, but is more 

 abundant in the less saline waters near the mouth of the Colorado 

 and the extreme upper parts of the bay, especially in the vicinity of 

 Dressing Point and above. In the latter locality it constitutes nu- 



