1918 | Packard: Molluscan Fauna from San Francisco Bay 239 
prised of pure sand as defined above (group 1). It is noteworthy 
that in both of these divisions gravel was lacking. In such places as 
in the vicinity of Point San Mateo an abundance of old shells offers 
ample support to the invertebrates requiring it. The middle division 
is predominately sandy. At 53.5 per cent of the stations within this 
region the bottom may thus be characterized. At 23 per cent it is 
prevailingly muddy and at 22.9 per cent it is predominately gravelly. 
Mud was encountered at only one station within this division, sand at 
8 or 20.5 per cent, and gravel at 7 stations. 
The correlation of these physical conditions with the distribution 
of certain of the mollusecan species may now be undertaken. Lists of 
the prevalent species for the different types of bottoms, as grouped 
above, have been prepared. These lists are based upon the occurrence 
of identifiable specimens of the different species, and not solely upon 
the occurrence of lying specimens. In some instances this procedure 
may lead to incorrect conclusions, for it is evident that dead shells 
may be transported considerable distances by various agents, of which 
tidal currents and hermit crabs are conspicuous. It will be noted in 
the table given below that several of the species oceur in six or more 
of the groups. These species whose distribution appears to be unre- 
lated to a particular type of bottom are: Cardium corbis, Macoma 
nasuta, Mya californica, Mytilus edulis, and Ostrea lurida. It is 
rather surprising to note that Mytilus edulis should be taken alive 
from a pure mud bottom. In the following table column 7 has undue 
weight, for the group has too few stations to have any real significance. 
A bottom of sand or of mud is not as favorable to a varied mol- 
luscan life as is one comprising two or more types of materials. This 
easily anticipated conclusion follows from a comparison of the average 
number of species per station represented by living specimens for a 
particular bottom group. These averages are two or less for mud 
(2.0) and sand (1.3), whereas they are about three for the other 
groups. This may also be shown by referring to the list of prevalent 
species for the bottom groups. There it is seen that the larger numbers 
are in the groups comprising two or more types of materials. This 
is presumably due to the greater number of environments at such 
localities. 
The records of the Survey indicate that a muddy bottom supports 
a larger number of mollusks than does a sandy or gravelly one. The 
average number of living individuals per stations for the muddy bot- 
toms (groups 1 and 2) is 38.4, whereas it is 20.6 for the sandy (groups 
