THE MICROPLANKTON OF PLYMOUTH SOUND. 135 
taken at the same time of day, between I] a.m. and 1 p.m., and the 
state of tide, wind, and weather noted. 
A great many species get through the meshes of even the finest nets. 
Those which are nearly always lost are the smaller Peridiniales, especially 
the Gymnodiniacee, the small Infusoria, with the exception of the Tin- 
tinnoidea, small flagellates (very few of which, however, appeared in our 
samples), Protozoa of various kinds and many of the smaller diatoms. 
On looking through the Plymouth records in the Fisheries Investigations 
we find an almost complete absence of all the very small Peridiniales, 
and with one exception (that of Gymnodinium lunula, which owing to 
its large size is conspicuous) an absolutely complete absence of Gymno- 
diniacee which confirms Lohmann’s statement that all were lost. Proro- 
centrum micans is almost absent from the tow nettings, here again in 
agreement with Lohmann, who found a large loss. Infusoria, except the 
Tintinnoidea are practically absent, and among the diatoms we find 
records of species such as Chetoceras curvisetum, which we have found 
the commonest species of this genus in the plankton, only represented at 
the most by the sign +, usually ror rr. At times it has appeared with 
us in quantity in the tow nets, but not nearly as frequently as in the 
water samples. Paralia sulcata is seldom to be found in the nets but 
is abundant in the water samples, and present nearly all the year round. 
In Gough’s lists it is usually marked r or rr, never by either Bygrave or 
Gough is it marked ce. 
Skeletonema costatum is another good example and one specially 
marked by Lohmann. Although sometimes recorded as ce for Plymouth, 
the few times it is thus marked bear no comparison with the numbers 
really contained in the water. This is a particularly abundant species 
here, and at Kiel it is shown to be in enormous numbers, most of which 
escape the net. The species of Nitzschia are also good examples, N. 
closterium and N. delicatissima particularly nearly always being lost by 
the net. 
- On the other hand, a good many of the larger species do not get into 
the water samples in anything like a representative number. For instance, 
the genus Biddulphia only appears very occasionally, when really it 
forms a most important part of the plankton at a certain season of the 
year. Streptotheca thamensis is another case; this species being very 
abundant at times in the tow nettings and only occurring in small num- 
bers in the water samples. The genus Rhizosolenia, although the relative 
abundance of the species is usually well shown in the water samples, is 
yet sometimes very ineffectually represented. For instance, in June 
Rhizosolenia Shrubsolei appeared in all the tow nets for two or three 
days, particularly on June 19th, especially in the medium net, 
