1908.] TJie British Freshwater Phytoplankton. 



171 



occurring in the plankton of the Faeroese lakes, 28 were observed in the 

 plankton of the Orkneys and Shetlands. 



Of a total of 178 species observed in the phytoplankton, 47'4 per cent, 

 were Desmidiaceae, 209 per cent, were Bacillarieae, and 9 - 6 per cent, were 

 Myxophyceae. 



IV. The Irish Lakes (West and South-west). 



. In 1906 we published an account of the plankton of some of the more 

 important lakes of the West and South-west of Ireland,* from collections 

 which we made in May, August, and September, 1901. 



In the summer the phytoplankton is greatly in excess of the zooplankton, 

 and the Entomostraca are only dominant in the early spring months. As in 

 the Scottish lakes, the phytoplankton of the lakes of the west and south- 

 west of Ireland is to a large extent Chlorophyceous, but the BacillarieEe, the 

 Peridiniefe and to some extent the Myxophyceae, are also conspicuous. The 

 latter are much more noticeable than in the Scottish plankton, especially 

 species of Anahcena, Oscillatoria, Gomjphosphceria, C'celosphcerium, and Chroo- 

 coccus. 



The Desmids are numerous, and include many of the characteristic western 

 types. There is a great abundance of Spondylosium pulchrum, var. planum, 

 Staurastrv.ru anatinum, St. jacidiferum, St. Arctiscon, St. pseudopelagicum, and 

 St. joaracloxum, var. longipes, but a curious absence of St. Ojohiura. The latter 

 is a feature of a large proportion of the Scottish phytoplankton, but we have 

 not yet observed it in the Irish lakes. In the lakes examined, the Desmid- 

 flora of the plankton was not quite so rich as that of some of the Scottish 

 lakes, but we think that an investigation of many of the smaller lakes of 

 Galway and Mayo would bring to light a phytoplankton not at all inferior 

 to that of the western Scottish lochs. 



Diatoms are very abundant, and form a relatively large part of the Irish 

 phytoplankton. Centric Diatoms are more numerous than in the Scottish 

 plankton, and are represented chiefly by species of Jfclosira and Cyclotella. 

 Taoellaria, Asterionella, and the narrow forms of Synedra are very conspicuous 

 in the Irish lakes. 



The Peridiniea? are generally abundant, and are represented by 10 species. 

 Much the most interesting of these is Peridinium limbatum (fig. 2), a charac- 

 teristic horned species which occurred in some of the small lakes of Galway. 

 It has only been found elsewhere in the United States. 



* W. and G. S. "West, "A Comparative Study of the Plankton of some Irish Lakes,' 

 ' Roy. Irish Acad. Trans.,'" vol. 33, sect. B, part 2, 1906. 



