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BULLETIN OF THE BUREAU OF FISHERIES 
ecologic standpoints) are fewer still. Until comparatively recently the identification 
of even these few could hardly be attempted by anyone not a specialist in the group, 
but thanks to Gran’s (1908) excellent synopsis, to Meunier’s (1910) beautiful figures, 
and to the fact that most of the important species are distinguished by rather precise 
characters, they are now no more difficult to name than are other planktonic groups; 
far less so, for instance, than the smaller copepods. A certain number of species, of 
course, are hardly to be determined except under most favorable circumstances. 
For example, certain members of the genus Chsetoceras are separable only when 
carrying their resting spores, but these are in the minority. It chances that most 
of the diatoms that are prominent numerically in the phytoplankton of our gulf at 
one time or another — for example, the members of the genera Thalassiosira and 
Rhizosolenia and most of the predominant members of the genus Chsetoceras — are 
characterized by such well-marked structural features that no one trained in sys- 
tematics in general and in the study of marine plankton in particular should experi- 
ence any unusual difficulty in referring them to their respective species by Gran’s 
(1908) tabular keys. What is required for this is close observation of small charac- 
ters, often under high powers of the microscope; but the technique is simple, amount- 
ing usually to nothing more than examination in water or in formalin — at most to 
the drying process employed by Gran (1908, p. 6) or to one of the modes of mounting 
described by Mann (1922). The complicated methods of cleaning, so valuable in 
the study of estuarine and bottom-living diatoms as a whole, are not essential when 
the object in view is merely the identification of the comparatively large and already 
well-known species of marine planktonic diatoms preserved in formalin as taken from 
the tow net. 
Since no attempt is made in the present paper to contribute to the systematics 
of marine diatoms, the nomenclature follows Gran (1908) strictly, except as noted 
below. The identification of the representative lists (p. 423) having been verified 
by Dr. Albert Mann, a leading student of the group, they are offered with some con- 
fidence, although the catches still await final examination. 
The peridinian element in the plankton of the gulf is represented chiefly by 
members of two genera — Ceratium and Peridinium — genera so unlike in appearance 
as to be separable at a glance ; and while a good deal of discussion has centered about 
the relationships, specific, varietal, or genetic, of the numerous representatives of 
Ceratium (which is usually the dominant peridinian in the Gulf of Maine), it is not 
difficult to refer the specimens in question to the proper subgroup — call it species 
or what you will — by the use of Paulsen’s (1908) recent synopsis. The following 
identifications follow him strictly. Fortunately the naked peridinians, 22 which are 
not only far more difficult to discriminate among but apt to be mashed past recog- 
nition in the nets, have never been prominent in our tows; in fact, never detected 
except for a brief period in the spring (p. 417). 
21 For descriptions and beautiful figures of these the reader is referred to Kofoid and Swezy’s (1921) monograph. 
