Dakin and Latarche — The Plankton of Lough Neagh. 41 



Fragilaria crotonensis is recorded as oceiirring commonly in the Central 

 European lakes. In Lough Neagh it is quite common. Now we find a striking 

 contrast to this in England and Scotland. In Messrs. West's records the genus 

 is absent from Ennerdale Water. The species Capucina is the only one present 

 in Wastwater, and is rare there, and this species is again the only one, and 

 rare in Loch Lomond. Thus there is a marked difference between the Scottish 

 lakes and Lough Neagh. F. crotonensis is recorded as occurring in Lough 

 Neagh by West, and also, in another paper, Proc. Eoy. Soc. 81 B, in Scottish 

 and English lakes. It is added, however, that " Fragilaria is somewhat rare, 

 and of the species which occur F. capuciim is the commonest." This does 

 not hold good for Lough Neagh. Kofoid records F. crotonensis from the 

 Illinois Eiver, where it is much less abundant than the species F. virescens. It 

 is predominantly a vernal planktont, having its maximum on May 10th. It 

 appear(id in February and disappeared after May l7th. In other years it was 

 only found in April, May, and December. 



Coscinodiscus. 



Coscinodiscus lacustris Grun. — The order Centricae, on the whole, includes 

 a small proportion of the Diatoms, and only a few fresh-water forms. The 

 family Coscinodiscaceae includes only three genera inhabiting fresh water in 

 the British Isles ; and it is noteworthy that all three occur in the plankton of 

 Lough Neagh — Cyclotella, Stephanodiscus, and Coscinodiscus. Only one 

 species of the latter genus occurs in fresh water in the British Islands, and 

 that not always in the plankton. Now, in Messrs. West's records of the 

 Scottish lakes and English lakes, Coscinodiscus lacustris is recorded as absent. 

 In Wesenberg Lund's great treatise on the plankton of the Danish lakes, no 

 mention is made at all of Coscinodiscus lacustris. It is not recorded by 

 Apstein for the lakes at Plon. 



Thus it is, perhaps, one of the most remarkable features of Lough Neagh 

 to find that Coscinodiscus lacustris occurs in very considerable quantities in 

 the plankton. The species seems to be present right through the year, with a 

 maximum in spring. The highest numbers occurred in March and April ; 

 the smallest in the months September to February. In the marine plankton, 

 Coscinodiscus is one of the most important diatom genera. Coscinodiscus is 

 not recorded by Kofoid from the waters of the Illinois. 



Stephanodiscus. 



The genus Stephanodiscus is represented by the species astraea in the 

 plankton of Lough Neagh. Like Coscinodiscus, this genus is not recorded 

 from the Scottish Lakes (except by Bachmann) or the English lake district, 



