144 MARIE V. LEBOUR. 
(6) 7’. decipiens (Grun.). Rare. 
(7) 7. subtilis (Ostenf.). This little species with its surrounding matrix 
occurred only rarely in 1916, although it was frequently noticed 
in 1915. 
(8) T. condensata (Cleve). Very rare. 
(9) Lauderia borealis Gran. An important part of the plankton from 
May to September, with intervals of scarcity. Helps largely in 
forming both diatom maxima. Rare from late autumn to early 
spring. Maximum in May. Its seasonal distribution agrees with 
Herdman’s records for Port Erin. At all depths, but largest 
numbers at the surface. Maximum of 1,079 in 50 ec. in May from 
the surface. 
(10) Leptocylindrus danicus Cleve. Fairly common from May throughout 
the summer, at other times very rare. 
CULO CZ 
Fic. 3.—Leptocylindrus sp. x 700. 
(11) L. sp. (Fig. 3). A small species which is like L. minimus Gran (1912), 
but never twisted as he describes; occurs fairly commonly in the 
summer plankton. There are seldom more than two cells in a 
chain and these are always quite straight. The two chroma- 
tophores, size and form agree with Gran’s species. 
(12) Guinardia flaccida (Castr.). Common at intervals from April to 
September, with a maximum in July. More common in the very 
fine tow nettings than in the water samples. The large numbers 
occurring at Port Erin in May and June (maximum in June) are 
noticeable. 
(13) Hyalodiscus stelliger Bail. Fairly common from October to February ; 
a winter species. At other times rare. 
Genus Coscinopiscus Ehr. 
All the species of Coscinodiscus we have found practically absent 
during the summer, which agrees well with Port Erin; although 
they continue through the year there except sometimes for one 
month, they are in very much smaller numbers through the 
summer. From September to May they occur at times abundantly 
and are common in the very fine tow nettings. 
