THE MICROPLANKTON OF PLYMOUTH SOUND. 147 
(21) R. robusta Norman. This is the only winter Rhizosolenia here. It 
begins in November and, although never common, continues 
till April. Chiefly in the tow nettings. Very seldom in the water 
samples. 
(22) R. Shrubsolei Cleve. Very common in May till the end of June, 
then dwindles and is rare in August, almost absent in the winter. 
(23) R. setigera Brightw. Very common in July and August, when it 
seems to take the place of R. Shrubsolev; rare in spring and autumn 
and almost absent in winter. 
(24) R. hebetata (Bail.) f. semispina (Hensen). Begins in May and is 
very common till the middle of August, after that is rare and 
disappears entirely in the winter. 
(25) R. alata Brightw. Begins to be common in June and continues 
till August, after that is only rarely found, although a few stragglers 
are present throughout the year. 
(26) Corethron criophilum Castr. Most frequent in October but never 
common. Absent for nearly the whole summer. 
Genus CH&#TOCERAS Ehr. 
Although scattered throughout the year, all the species occur 
chiefly in the spring, summer and early autumn, forming an 
important portion of both maxima. A very large maximum in 
May (Fig. 5) agrees with the Port Erin records, but the autumn 
maximum in August is small, not amounting to more numbers than 
in March. This rise in March is partly due to numbers of C. 
densum, the maximum number of that species in the water 
samples. This species, however, is large and, like C. boreale, does 
not get much into the water samples. Chetoceras curvisetum, 
which is much the commonest species found, shows two well- 
marked maxima, a large spring and a small autumn maximum, 
these agreeing with the Port Erin records for the genus. The fact 
that on several days in early autumn no Chetoceras species were 
seen in the water samples brings the average for the month down. 
(27) Chetoceras densum Cleve. Frequent in the tow nettings, but too 
large to be found much in the water samples. Present most of 
the year except at times in the summer. 
(28) C. convolutum Castr. From spring to autumn, sometimes abundant 
(29) C. danicum Cleve. Rare, at intervals through the vear 
