69 



In studying these curves, it must be remembered that only their variations are of particular 

 importance. The absolute density is not so much so, except for comparison with that of other 

 localities; but the variations are important if by them we can account for the failure either of the 

 propagation or attachment of the young oysters. The curves will show certain irregularities, due 

 to either the variations in depth, or to the tides having changed from flood to ebb, or the reverse, 

 on one side of the Sound at a time differing from that on the other. 



As will be seen, however, the greatest variations in each month are in Tangier Sound, on the 

 eastern side, where the influence of the rivers is felt to greatest extent. In Pocomoke Sound the 

 greatest variations appear to be on the western side, and I assign as a reason for this that the 

 influence of Guilford and Mesongo Creeks is of small importance compared with that of the Poco- 

 moke Eiver, the current from which sweeps along the northern and western parts of the Sound. 

 The curves show that the variation is very slight, except on the September sections. 



The second series of curves, those showing the monthly change of mean densities, indicate that 

 the maximum change on each section was about the 1st of September; that the variations in Poco- 

 moke Sound were much greater than in Tangier Sound; and that the maximum changes were at 

 the head of each Sound, and the Variations diminished towai'ds the entrances. 



The third series of curves shows the same when all the observations in each Sound are assem- 

 bled, but with this difference — while the density in Tangier Sound was greater on October 1st than 

 at any other time, in Pocomoke Sound the influence of the river was still felt; and notwithstanding 

 the diminished temperature the density on the 1st of October was less than on the 1st of August. 



The fourth series of curves shows the difference in density between the upper and lower sec- 

 tions in Tangier and Pocomoke Sounds in each mouth, and indicates that the density of the water 

 is considerably greater over the lower beds than on the upper. 



The maximum density found during the summer was on section 3, in October, and was 1 0166. 

 The minimum density was found in section 5, in September, and was 1.0005. 



By referring to the curves showing monthly changes of mean densities, it will be seen that in 

 only one case, that of section 5, does the density become less than 1.0100, and that even on this 

 section it is evident that this was not the normal condition of the water. There were heavy freshets 

 in the Pocomoke Eiver during the latter part of August and during September, which accounts 

 for the slight density as shown by the curves. Mr. Barroll was informed by the inhabitants of the 

 vicinity of the mouth of Pocomoke Eiver that these freshets had tilled large numbers of oysters, 

 both on the natural and planted beds. An insi)ection of the other curves shows that the variation 

 of density on successive tides was not much greater on this section than on the others, and as the 

 oysters elsewhere in the Sounds did not appear to suffer from the effects of these variations, I am 

 of the opinion that the fluctuation was not sufficient to affect the mature animal, but that iu this 

 case the water continued fresh, or practically so, for too long a period, thus killing the oysters by 

 endosmose. 



From the observations of density, then, it may be assumed that the density of the water, iu 

 these localities at least, cannot fall below 1.01 for any protracted period without destroying the 

 oysters. Whether the changes in density affect the spatting can only be ascertained by continu- 

 ing the observations for a number of seasons, or by direct experiment with the spat artificially 

 raised. 



INCIDENTAL INFORMATION. 



During the summer I have examined, under the microscope, 374 oysters, of which 212 were 

 females and 162 males, or the percentage of females was 0.56. The oysters were not all examined 

 at the same time, nor were they all from the same bed, but the percentage in each of the lots 

 examined, twelve in number, does not vary greatly from the percentage given above. I am of the 

 opinion, therefore, that about 60 per cent, of the oysters in a commuuity are females. A larger 

 number of observations is, however, desirable. 



In making these examinations I have never seen both ova and spermatozoa in the same animal, 

 though I have made many careful observations in order to detect the presence of either. I have 

 also carefully examined the gills and mantles of a large number, and have never found an embryo 

 oyster within the shell; and as these observations were made during the spawning season, I do 



