Roper, on the Diatomacem of the Thames. 
69 
of the current is so great, that the germs of these minute 
organisms have not time to increase and multiply as they do 
in more sluggish streams, flowing for a long distance through 
alluvial deposits. 
A similar occurrence of marine Diatomacece at a considerable 
distance from the sea has, however, been noticed by Professor 
Bailey, in America, who, in his ' Microscopical Observations 
on South Carolina and Georgia,' published by the Smithsonian 
Institution, expresses the surprise with which he found in 
Lake Monroe, 200 miles from the mouth of the St. John's 
river, specimens of Amphiprora constricta, Odontella poly- 
morpha^ and Navicula elongata, which he considered decidedly 
marine, and which had often occurred to him on the shores of 
the Atlantic. 
I now proceed to give lists of the species from different 
localities in the Thames, placing those from the Isle of Dogs 
first, and comparing them with the forms from Hammersmith 
and near Gravesend ; and though I have been unable at pre- 
sent to examine the deposits of the two latter localities so as 
to give more than a general view of the species, yet these are 
sufficiently well marked to show the distribution of those 
peculiar to marine and fresh water. 
In all the localities many species of Melosira, Odontidium^ 
and other genera occur, which, from the want of good figures, 
I have been unable to name. The well-marked frustules of 
those figured in the first volume of the Rev. William Smith's 
valuable synopsis have been easily recognized, from the ex- 
tremely accurate figures there given. In all cases where any 
doubt existed, I have referred to slides of the species authen- 
ticated by Mr. Smith himself. In some few instances I am 
indebted to his kind assistance, and also to his able coadjutor, 
Mr. West, for the determination of forms I was unable 
satisfactorily to identify, and in a few others I have depended 
on the figures of Kiitzing's work on the Diatomacece. One if 
not two species of Dictyocha occur in the mud from the Isle of 
Dogs, but I have excluded them from the list, as there appears 
some doubt if they can be correctly referred to the same order. 
Marine and Brackish Water Species from the Isle of Dogf^. 
1. Epithemia sorex 
2. „ musciilus 
3. Amphora affinis 
4. hyalina 
5. Cocconeis scutellum 
0 diaphana 
7. Coscinodiscus radiatus 
8 „ eccenti'iciis 
9. Eiipodiscus argus 
10. fulvus 
11. radiatus 
12. „ sculptus 
13. Actinocycliis iindiilatus 
14. „ sedepariua ? 
15. Triceratium favus 
16. „ striolatum ? 
h 2 
