THE MICROPLANKTON OF PLYMOUTH SOUND. 151 
(49) B. sinensis Grev. Not so abundant as the other two but fairly 
common, continues until the end of May. 
(50) B. favus (Ehr.) v. Heurck. Rare, February and April. 
(51) B. alternans (Bail.) v. Heurck. Rare, October and early spring. 
(52) Bellerechia malleus (Brightw.) v. Heurck. Rare, September. 
(53) Lithodesmium undulatum Ehr. Common from August to October, 
rare at other times. 
(54) Ditylium Brightwelli (West) Grun. Appears and disappeais peri- 
odically from January to September. In March and September 
very common in the tow nets. 
(55) Fragillaria sp. Sometimes present in long strings in summer. 
(56) Thalassiothrix nitzschioides Grun. Common from September to the 
end of April. A winter species In summer rare or entirely absent. 
(57) Asterionella yaponica Cleve. Important in the late summer. Occurs 
in single groups rarely at intervals from October to the end of 
June, then suddenly becomes very common in July, rising to 
over 478 per cc. at the end of the month, abundant in August and 
gradually dwindles through September. Present in the tow 
nettings as well as the water samples. This seems to be erratic 
in its appearance as Gough records it as cz. in April and May (as 
A. glacialis). The largest numbers occur at 5 and 7 fathoms, 
maximum at 5 fathoms. 
(58) A. Bleakeleyi W. Smith. Only occurred twice, November and 
December. 
(59) Lycmophora Lynbergi (Kiitz) Grun. Rare, at intervals through the 
year. 
(60) Grammatophora serpentina Khr. <A littoral species, rare. 
(61) Acnanthes longipes Ag. Rare, in tow nettings, autumn and early 
spring. 
(62) Navicula membranacea Cleve. Fairly common from July to 
November. 
(63) N. sp. Many species of Navicula occurred through the year which 
were not identified. 
(64) Pleurosiqma sp. Several species occurred through the year. 
(65) Mastiglova sp. Occurred at intervals in such numbers as to materi- 
ally influence the records. The large numbers are always at the 
surface, although in July and August it occurs at all depths. 
