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The Peridiniales of Plymouth Sound from the Region 
beyond the Breakwater. 
By 
Marie V. Lebour, M.Sc., 
Assistant Lecturer in Zoology, Leeds University. 
Temporary Naturalist at the Plymouth Laboratory. 
With Figures 1-14 in the text. 
THE following list includes all the Peridiniales identified in the plankton 
throughout the year from September, 1915, to September, 1916, from 
the water samples, details of which will be found in another paper in 
the same journal (p. 133). Also from the plankton in the tow nets in the 
same year and a portion of the summer 1915. As is shown in the above- 
mentioned paper, the summer is the time for nearly all the Peridiniales, 
June being the maximum month. After October very few are seen in 
the water samples, although in the tow nettings the larger and stronger 
forms, such as Ceratiwm and Peridinium, are still present. 
The new and least-known forms belong to the Gymnodiniacee, which 
have no cellulose sheath. Perfectly transparent and extremely thin 
cases, however, are often seen which may be close fitting or many times 
larger than the gymnodinian. Pouchetia and various species of Cochlo- 
dinium are instances of this (see Plate IT, Fig. 14). The Gymnodiniacee 
are perhaps the most interesting of the Peridiniales, as many of them 
obtain nourishment holozoically and often the food can be determined. 
Throughout June the flagellate Phaocystis pouchetii was excessively 
abundant, and this furnished food for many gymnodinians (e.g. Gymmno- 
dinvum rhomboides and G. triangularis, see Plate I, Figs. 6 and 7). 
Division stages in this group are often seen which in the genera Gymno- 
dinium and Spirodinium take place usually, if not always, in the free 
state and not in capsules as in Pouchetia and others (Pouchet, 1885 ; 
Dogiel, 1906). 
Although no special investigation has been made closer inshore, the 
examination of a few samples of water show that many of the species 
occur near the land, such as Gymnodinium and Spurodinvum species 
and Dinophysis, besides several species of Peridinium. 
