‘ 
1918] Packard: Quantitative Analysis of Molluscan Fauna 309 
in sheltered localities on muddy or sandy beaches. It thrives under 
various conditions of temperatures and salinities. The extensive mud 
flats of San Francisco Bay afford a very congenial habitat for this 
exotic form, as is attested by its phenomenal increase since 1881, when 
it was first reported from this region. The Survey record shows that 
this species now has a general distribution within the bay, being 
especially abundant on the extensive tidal flats of the upper and lower 
divisions. 
An excellent account of the developmental history and economic 
importance of this clam may be found in the reports of the Massa- 
chusetts Commissioners on Fisheries and Game. Since no detailed 
work has been published regarding this particular species on our coast, 
Belding’s conclusions will be assumed to apply in general to our local 
forms. The following notes are drawn freely from the papers pub- 
lished by the Massachusetts Commissioners on Fisheries and Game 
(1916). 
As is well known, this mollusk burrows deeply in the soil, lying 
at a depth of from six to twelve inches. When the tide is out the 
siphon is generally partly retracted, leaving an elliptical hole in the 
sand, but upon the return of the tide the siphon expands and a current 
of water is set up through the incurrent and execurrent tubes. The 
clam once having established itself and having grown to a length of 
about one and one-half inches, seldom moves, unless crowded out of 
its hole by more vigorous neighbors. 
Belding and Lane show that after fertilization the larva passes 
through the well known stages leading up to the veliger, which is 
characterized by a thin shell. This stage is reached in about twenty- 
four hours after fertilization, the organism passively floating at or 
near the surface of the water. A few days later it develops a pro- 
dissoconeh and a ciliated foot, when it settles to the bottom and at- 
taches itself to a suitable support by means of a byssus. It develops 
rapidly and soon acquires the burrowing habits of the adult. 
In favorable localities on the Atlantic Coast a length of 30.5 milli- 
meters (114 inches) is attained by the end of three and one-half 
months. Belding and Lane (1916, pl. 9) claim that a clam that has 
reached a length of 25 mm. at the end of six months will measure 
70 mm, at 114 years, or 81.9 mm. at 21% years, or 90.7 mm. at 3% 
years. Such a growth expressed in terms of volume is equivalent to 
an increase from 1 to 23 bushels at 114 years, 36.9 at 214 or 47 at 314 
years. This clam reproduces on the Atlantic Coast at two years of age. 
