330 bulletin: museum of comparative zoology. 



because one adult Calanus is worth many Centropages or Pseudo- 

 calanus in food value, though the latter are an important food for 

 fish fry. It is the Calanus swarms which form the chief copepod con- 

 stituent of the food of mackerel, pollack, and probably of the shad 

 which summer in the Gulf; and for Calanus as for the volume of plank- 

 ton, the richest parts of the Gulf were off Massachusetts Bay and off 

 Mt. Desert Rock (Stations 10092, 10100), with a third prolific area 

 off Chatham detected by Captain McFarland. 



MiCROPLANKTON. 



The microplankton of the cruise will be treated later in special 

 reports. But it is worth while to give a brief account of the distribu- 

 tion of general plankton types here, because of their bearing on general 

 oceanographic problems (Fig. 82) . They fall into four general types, 

 which may be called "Ceratium," "diatom," "mixed" (a mixture of 

 the two), and a tropical type characterized by the presence of con- 

 siderable amounts of Trichodesmium. Of course these are not 

 actually distinct, grading into one another; but they group sufficiently 

 well to be treated in this way. To take the rarer types first, tropical 

 plankton (the "Desmo Plankton" of Cleve) was encountered only 

 once, in the inner edge of the Gulf Stream (Station 10071) where 

 the rather scanty catch consisted chiefly of Ceratium macroceros, and 

 of Trichodesmium, with an occasional diatom (Rhizosolenia) . Dia- 

 tom plankton was encountered in three distinct regions; on George's 

 Bank (Station 10059); off the mouth of Chesapeake Bay (Stations 

 10075, 10077, 10078) and in the northern part of the Gulf of Maine 

 near Mt. Desert (Stations 10099, 10101). 



The species composing these diatom swarms were quite different 

 in these three regions. On George's Bank the mass, which was rather 

 rich, consisted chiefly of a species Guinardia, besides such forms as 

 Eucampia zoodiacus, Rhizosolenia stolforthi, and R. styliformis, practi- 

 cally a pure diatom haul, except for an occasional Peridinium and 

 Ceratium. The diatom swarm off Chesapeake Bay consisted chiefly 

 of various species of Chaetoceras (among them C. decipiens and C. 

 contortum) with smaller numbers of Rhizosolenia, Leptocylindrus, 

 and Thalassiothrix. And at the mouth of the Bay the haul was 

 chiefly Rhizosolenia. 



The diatom plankton found in the Gulf of Maine is difficult to place 

 because it was chiefly debris, and evidently moribund. But fragments 

 of Rhizosolenia and Chaetoceras decipietis, with other species of Chae- 



