A comprehensive review of the taxonomy of the genus Prorocentrurn (which now includes Exuviuella) has been carried out for the first time since Schiller's monograph of 1933-7. All 64 valid taxa have been considered, besides a large number of illegitimate names. In view of the great intraspecific variation which takes place in these organisms, it is considered that the majority of the described taxa relate to varieties or aberrant forms. It is proposed to reduce the number of species from 64 to 21, which fall into five fairly distinct groups A key to the species is provided and new descriptions and figures are given. The possible phylogenetic relationships within the genus are considered. CONTENTS 103 9 14.P. gracile b Spine very long and narrow; posterior half of cell abruptly narrowed and sometimes twisted . . . . . . 13. P. urcuatum Arranged in order of the first letter of the specific name. (E = Exuviuella; P = Prorocentrum)P. adriaticum = 17. P. vaginulum E. aequatorialis = 21. P. balticum E. almonda Nie (1947), nomen (?) E. antarctica = 1. P. aporum E. aperta Schiller (1928), inadequate description E. apora = 1 . P. aporum P. aporum ( l ) , p.107 P. arcuatum (1 3). p. 1 1 4 E. baltica = 21. P. balticum P. balticum (21), p. 118 P. bidens = 7. P. compressum P. blatta = 1 3 . P. arcuatum P. brochi = 18. P. maximum E. caspica = 3. P. lima E. cassubica = 2. P. cassubicum P. cassubicum (Z), p. 108 E. cincta = 3. P. lima E. circularis Gaarder (in Braarud e t al., 1953). E. compressa = 7. P. compressum. P. compressum E. cordata = 20. P. cordatum P. cordatum (20), p. 118 P. cordiformis = 19. P. minimum P. c o m u t u m = 9. P. rotundatum P. curvatum Kofoid (1907), not a Prorocentrum E. dactylus = 5. P. dactylus P. dactylus (5), p . 109 P. dentatum (16). p. 116 P. diamantinae = 14. P. gracile E. elongata = 7. P compressum P. gibbosum = 1 3 . P. arcuatum P. gracile (14), p, 114 E. grani = 1. P. aporum P. hentschelii = 14. P. gracile E. intermedia Bursa (1962). nomen E. kapuiuikii Bursa (1962), nomen P. lebourae = 7. P. compressum E. lenticulata = 7. P. compressum P. levantinoides = 12. P. micans E. lima = 3. P. lima P. lima (3). p. 109 nomen (7), p. 1 1 0 P. macrurus = 14. P. gracile E. magna = 8. P. magnum P. magnum (8). p. 111 E. marine-lebouriae = 19. P. m i n i m u m E. marina = 3 . P. lima P. marinurn = 3. P. lima P. m a x i m u m (18). p. 117 P. mexicanum = 18. P. m a x i m u m P. micans (12). p . 112 P. m i n i m u m (19). p. 117 E. missolongica Athanassopoulos (1931 ), in-P. missolongicum Athanassopoulos (1931), in-P. monacense = 16. P. dentatum E. monospina Hasle (1969). nomen P. nanum (6), p. 110 E. oblonga = 7 . P. compressum P. obtusidens = 16. P. dentatum P. obtusum = 18. P. m a x i m u m E. ostenfeldii = 3. P. lima P. ouale = 18. P. m a x i m u m E. o v u m = 4. P. o v u m P. o v u m (4), p. 109 E. pacijica = 20. P. cordatum P. pacificurn = 12. P. micans E. para-compressa Athanassopoulos (1931), ? not a E. peisonis = 20. P. cordatum i? pomoideum = 21. P. balticum P. p r o x i m u m = 18. P. m a x i m u m ...

On the basis of SEM studies of plankton material from around the British Isles and from several other parts of the world a detailed revision has been carried out involving over 20 species which have been ascribed to Gonyaulax but excluding the toxigenic species now attributed to Alexandrium. As a result the genus is redefined as having the tabulation Po, 3', 2a, 6", 6c, 4-8s, 5'", Ip, 1"" and 17 species are reported which clearly fit this formula. The old genera Amy lax and Protoceratiwn have been resurrected to accommodate organisms with a differing tabulation and the fossil genus Lingulodinium has been emended to include its motile thecate stage previously known as Gonyaulax polyedra. The relationships of the species are discussed and the revision is accompanied by numerous micrographs to illustrate the major characteristics of the species.

The distribution of the dinoflagellate genus Ceratium Schrank (Dinophyceae) in the North Atlantic and adjacent seas was studied by a combination of new observations on a large number of plankton samples collected from the northeastern Atlantic and North Sea, data from cruises off the east coast of North America and Caribbean Sea, and reports in the literature of the past 90 years. Seventy species were recorded, and their distribution was examined by several methods. Distribution maps were plotted for all species, and from these the ranges of temperature tolerance were derived. The 240 sets of data, which took the form of lists of species present in 5° latitude / longitude blocks obtained from the new work and the published material, were analyzed by clustering and ordination multivariate techniques using the programs Twinspan and Decorana. Analysis of the individual species showed that surface water temperature is the most important factor determining distribution and the number of species in a particular area. Warm water and /or low latitudes have many more species than cold waters and/or high latitudes. For example, at 5°N there are on average 23 species per block, whereas at 60° N there are only 8 species. On the basis of this work, the Ceratium species are divided into Group 1, Arctic‐temperate species normally only found in water of less than 15°C; Group 2, cosmopolitan species, which are found virtually everywhere and are the species most likely to form blooms or “brown water”; Group 3, intermediate species, which extend into neither the coldest nor the warmest water; Group 4, temperate‐tropical species, which have a lower temperature boundary of 5°–12° C; Group 5, warm‐temperate‐tropical species with a lower temperature boundary of 14°–15°C; and Group 6, tropical species, which are rarely found in water of less than 20° C. Analysis of the sample sites also confirmed the predominant influence of temperature, and the Atlantic Ocean was divided into four biogeographical zones of which the boundaries follow isotherms of surface water temperature. Zone 1 consists of the Arctic and subarctic area, with the southern boundary closely following the 10° mean annual temperature (MAT) line. Zone 2 is an intermediate or cold‐temperate zone, of which the southern boundary follows the winter 10° C MAT isotherm or the similarly placed summer 15° isotherm. Zone 3 is the warm‐temperate or subtropical zone, which is very broad. The southern boundary closely follows the 25°C summer isotherm. Zone 4 is the tropical zone, where water temperature is never likely to be much less than 23°C. These findings are discussed in relation to experimental work and environmental observations. We suggest that the genus Ceratium provides an excellent tool for defining ocean currents and temperature changes and may become of value in studies of global change.

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The structure of the theca of some 30 marine and freshwater members of the Dinophyceae has been examined by light and electron microscopy. The basic arrangement of membranes, an outer continuous plasma‐membrane beneath which lies a single layer of flattened vesicles, is the same in all flagellated forms. The periplast of coccoid and encysted forms is somewhat different. Thecae have been found to fall into eight reasonably distinct categories as follows: (1) Outer membrane underlain by irregular vesicles. (2) Membrane underlain by close‐packed vesicles. (3) As Group 2 but with plug‐like structures attached to the inner membrane of the vesicles. (4) As Group 2 but with thin plates within the vesicles and all plates of more or less similar shape. (5) As Group 4 but plates have slight overlap. (6) Similar to Group 5 but plates are reduced in number and can be individually recognized. (7) As Group 6 but plates have elaborate flanges and their surfaces are covered by reticulations. (8) Theca mainly consisting of only two plates. It is suggested that the thecal types could from the basis for some revision of the taxonomy of the class.