18 ROPER, ON BRITISH MARINE DIATOMACEA. 
peculiar forms that have occurred to myself within the last 
two years. 
The mud of tidal harbours, and the creeks and pools on the 
banks of estuaries, such as occur in the Thames, Poole Har- 
bour, &ec., have been the chief source of supply of our present 
marine flora of this class. But a large number of the more 
interesting species are only to be obtaimed by dredging or 
collecting the various species of filamentous marine Algze at 
the lowest spring tides. These should be gathered in consi- 
derable quantity, thoroughly washed, and left for some short 
time in water, so that all the Diatomacez may become de- 
tached. The sediment must then be allowed to subside, a 
portion of it bemg preserved for the examination of any new 
or interesting forms in a living state, and the remainder 
treated with acid in the usual way. The sand and any re- 
mains of the Algze not dissolved by the acid may then be 
removed by subsidence, on the plan recommended by my 
friend Mr. Okeden, in the ‘ Microsc. Journal,’ vol. i, p. 158, 
which is preferable to that of Dr. Munro, described at page 
241 of the same volume, as it is impossible to prevent the 
admixture of gatherings from different localities by this 
process, though when that is not an object it has some 
advantages. 
From an examination of the species described in Professor 
Smith’s ‘Synopsis,’ I find that out of 455 species included in 
that work as indigenous to Great Britain, 231 are from fresh 
water, 82 occur in brackish water, and 142 are marine; and 
of this latter number 72 have been collected from Poole, 
Pevensey, Hull, and the Thames, whilst 10 were obtained 
from molluscs, and only 6 are described as dredged in deep 
water. That this gives a very imperfect notion of the nume- 
rous species to be found at a considerable distance from the 
shore is shown by the examination of the gatherings in 
which the greater part of the new species now to be described 
were found. 
The Caldy gathering, which was made in five to six fathoms 
water, contains many rather rare and interesting forms, in- 
cluding Coscinodiscus concinnus, Biddulphia Baileyii and 
rhombus, EHucampia zodiacus, Nitzschia spathulata, and Melo- 
sira Westii, and I have altogether met with sixty-six species 
described by Professor Smith. In the Lyme Regis gathering, 
from a depth of five to eight fathoms, Synedra undulata, 
Amphora costata, Campylodiscus Hodgsonit and Ralfsii, Na- 
vicula crabro, and Rhabdonema Adriaticum occur, and I have 
already found seventy-nine species included in the ‘ Synopsis.’ 
In addition to the new species and varieties which I now 
proceed to describe, there are numerous other forms in both 
