1918] Packard: Molluscan Fauna from San Francisco Bay 243 
A comparison of the lists of the species obtained from the several 
types of bottoms shows that most of the species were taken from more 
than one type of bottom. This indicates that the mere occurrence 
upon a particular type of bottom does not necessarily prove that such 
a bottom represents its normal habitat. The relative abundance per 
unit of area for the different types of bottom probably more nearly 
indicates the optimum habitat of a given species. Thus the average 
per haul for each species should give some clew to the most favorable 
environment of that species. Cardiwm corbis, Macoma nasuta, Mya 
arenaria, and Zirfaca gabbi are, according to such reasoning, predom- 
inately mud dwellers. On the other hand, Mya california, Macoma 
balthica, M. inquinata, Ostrea lurida, and Thais lamellosa predominate 
on the sandy bottoms. Here again the conclusion drawn from the quan- 
titative work does not agree with that based upon the qualitative hauls. 
It has already been stated (p. 239) that Cardium corbis, Macoma 
nasuta, Mya california, and Ostrea lurida do not show a distribution 
pattern that appears to be related to any particular type of bottom. 
In this case it is not unlikely that the latter conclusion is the more 
nearly correct. 
A standard specimen curve, showing the number of living indi- 
viduals obtained at the dredging stations that are the equivalent of 
the hydrographic stations of the regular series, has been made in order 
to compare it directly with the temperature and salinity curves pub- 
lished by Sumner et al. The specimen curve shows little relationship 
with the mean annual salinity curve. The seasonal range of salinity 
eurve (Sumner et al., 1914, p. 68, fig. M) shows a certain correspond- 
ence with the specimen curve, indicating that there is in general an 
increase in the number of living individuals per haul with decrease 
in the annual range of salinity. One of the six curves showing the 
mean salinity for the different periods of the year suggests that the 
low salt concentration during the period from April 23 to May 6 
may possibly be a factor limiting the abundant molluscan life to the 
middle and lower divisions of the bay. 
This same specimen curve when compared with the published tem- 
perature curves for the same stations shows that the regions where 
the annual range is high are low in the number of mollusks. It also 
indicates that the cooler regions of the bay during the summer months 
support a greater number of specimens per unit area. However, these 
apparent relationships are not considered as being necessarily causal 
ones. 
