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 excurrent 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- 

 dissoconch 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 (11/4 inches) is attained by the end of three and one-half 

 months. Belding and Lane (1916, pi. 9) claim that a clam that has 

 reached a length of 25 mm. at the end of six months will measure 

 70 mm, at 1% years, or 81.9 mm. at 21/2 years, or 90.7 mm. at 3I/2 

 years. Such a growth expressed in terms of volume is equivalent to 

 an increase from 1 to 23 bushels at l^^ years, 36.9 at 2i/^ or 47 at 314 

 years. This clam reproduces on the Atlantic Coast at two years of age. 



