STUDIES OF THE OYSTER DRILL 
91 
remained constant. For this reason the animals were not introduced into the jars 
until the second day. 
Table 3. — Effects oj varying salinity on Urosalpinx cinerea from Beaufort, N. C., summer, 1928 
Items 
Experiments 
No. la 
No. 2a 
14.45 
90 
85 
10 
26 
8. 6- 8. 8 
No. 3a 
Salinity at which 50 per cent of drills were killed 
12.81 
95 
16.89 
90 
10 
26 
8.4- 8.8 
16.00 
70 
17.43 
100 
15 
24 
Percentage of deaths after 10 days. - 
Salinity at which at least 85 per cent of drills survived 
Percentage of survival after 10 days 
Percentage of deaths in control 
Average temperature for period (°C.) 
pH for period. 
Before giving the more detailed Beaufort experiments made during the spring 
and su mm er of 1929 certain conclusions derived from the above results should be 
given. In order to do so a word of explanation is needed concerning lines 1 and 
3 in Tables 2 and 3. Line 1 gives the experimental salinity at which at least 50 
per cent of the drills died; line 3 gives the next highest salinity in the experiment 
at which not more than 10 to 15 per cent of the animals died. The author has 
arbitrarily taken the mean of the figures in lines 1 and 3 and called this the salin- 
ity death point. It is believed that the nature of the experiments and results justify 
such a procedure. This method gives as close an approximation to the salinity death 
point as can be gotten even by reducing the intervals between the salinities used. 
If the results given in Tables 2 and 3 are analyzed according to the method just 
indicated the results obtained are these: Urosalpinx cinerea collected from Hampton 
Roads in two regions having respectively the following average summer salinities, 
15 and 20 parts per mille, have a salinity death point of approximately 12.5 parts 
per mille. On the other hand animals collected from Beaufort where the summer 
salinity is well over 30 parts per mille have a higher salinity death point; that is, 
15.6 parts per mille. 
The discrepancy between the death-point salinity and the salinity below which 
drills do not occur in the field at Hampton Roads can easily be explained after a 
study of the variations in the salinity of this region occurring throughout the year. 
During March, April, and early May (1927) the salinity at Craney Island dropped 
below 12 parts per mille for several days, although during the summer and winter 
months the salinity averaged over 17 parts per mille (Federighi, 1930a). Thus, 
although the survey during June, 1927, showed that the drills are not found below 
a salinity of 15 parts per mille, this is not the minimum salinity for survival, since 
during the spring the salinities of these areas fall as low as 12 parts per mille — a 
figure sufficiently close to the experimentally determined salinity death point (12.5 
parts per mille). 
During the summer of 1929 detailed experiments were conducted to check the 
observations made in 1927 and 1928. For this purpose a total of 3,290 animals were 
used at salinities ranging from 8 to 35 parts per mille. Of this total number 150 
drills were from Hampton Roads (Series VIII), the remainder (3,150) were collected 
at Beaufort. The procedure was essentially the same as that employed for the pre- 
liminary Beaufort experiments. In these studies 50 drills were placed in each jar 
which contained 2 gallons of sea water of the desired salinity. The water was kept 
48390—31 2 
