80 OYSTER BOTTOMS IN MATAGORDA BAY. 



P. intermedium W. Smith (pi. xin, fig. 7), length 19 c. d. m., 

 breadth 2 c. d. m., thickness 1 c. d. m., volume=0.6 (lXbXt)=25 

 cubic c. d. m. 



P. tenuissimum W. Smith, length 30 c. d. m., breadth 1.8 c. d. m., 

 thickness 1.2 c. d. m., volume=0.75 (lXbXt). 



P. angulata major W. Smith, length 20 c. d. m., breadth 3.5 c. d. m., 

 thickness 1 c. d. m., volume=0.6 (lXbXt)=45 cubic c. d. m. 



Melosira sp., Pyxilla sp., and five or six others of occasional 

 occurrence. 



In determining the actual food of the oysters, 10 specimens be- 

 tween 4 and 4 J inches in length were selected from each locality, the 

 shells carefully opened, and the contents of the stomachs removed as 

 completely as possible by means of a pipette. To the stomach con- 

 tents of each lot sufficient water and formalin were added to raise the 

 volume to 5 c. c. and the number of diatoms per oyster was com- 

 puted by the Rafter method, before alluded to. The results for most 

 of the principal reefs are exhibited in the table on page 72. For pur- 

 poses of comparison there is shown in each case the number of dia- 

 toms per liter of surrounding water as determined by the average of 

 all counts of water specimens taken on the bed and in its immediate 

 vicinity. The water specimens on the reef were usually taken at the 

 same time as the oj^sters, but owing to the exigencies of the work the 

 specimens over the neighboring bottoms were sometimes taken several 

 days before or after. This may explain some of the minor incon- 

 sistencies of the table. 



It will be observed that all of the species found in the water enter 

 more or less into the dietary of the oyster, but that of the commoner 

 forms the smaller, more active organisms, like Navicula didyma, 

 Synedra commutata, and Prorocentrum, are taken up in larger pro- 

 portion than the larger, less motile species, like the Goscinodisci and 

 Melosira distans. It would appear, too, that long spinous species like 

 Pleurosigma tenuissimum would be practically valueless as food even 

 were they more abundant, probably owing to their entanglement in 

 the cilia of the gills, palps, etc., which would retard their movement 

 toward the mouth. 



The most astonishing development of the endeavor to make a volu- 

 metric estimate of the o}^ster's food was the small quantity found in 

 the stomachs. Numerically the results accord fairly with the writer's 

 previous experience and with the results obtained by other investiga- 

 tors, the methods being in general the same in all cases ; but the volu- 

 metric results showed the average stomach content of all oysters 

 examined to be about one-eighth cubic millimeter, less than one-tenth 



