382 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 (190S) 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 

 (190S) 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). 



11 For descriptions and beautiful figures of these the reader is referred to Kofoid and Swezy's (1921) monograph. 



