Roper, on the Diatomacece of the Thames. 71 
rjermanicus and Actiiiocychis undulatus, probably identical with 
Actinoptychus senarius. This would seem to show that though 
the general results were similar, yet from some peculiarity, 
either in the water or the distribution of these minute 
organisms, the species abounding in the rivers of the north of 
Europe are marked with a distinctive character from those 
found in the Thames. The prevailing form in the Elbe ap- 
pears to be the Actinocyclus, and its allied genus Actinoptychus 
of Ehrenberg, of which he enumerates no less than fourteen 
species out of the thirty-four marine forms that he recognised. 
On comparing with the foregoing list from the Isle of Dogs, 
the species which occur in the mud at Hammersmith and near 
Gravesend, it appears, that though a few marine forms are 
still found at the former locality, yet the preponderance of 
fresh- water species is very great ; whilst at the latter the marine 
and brackish water species, with a few exceptions, alone 
occur. 
The following lists include all I have at present met with 
from those localities : — 
Marine and Brackish Water Species 
1. Epithemia musculus 
2. Cocconeis scutellum 
3. Coscinodiscus eccentricus 
4 . , , radiatus 
5. „ marginatus? 
6. EupodiscTis argns 
7. crassus 
Actinocyclus undulatus 
sedenarius? 
Triceratium favns 
,, alternans 
12. Campylodiscus cribrosus 
13. Surirella ovata 
14. gemma 
15. „ fastuosa 
16. Tryblionella acuminata 
17. marginata 
18. „ punctata 
19. Nitzschia sigma 
20. „ dubia 
9. 
10. 
11. 
! from the Thames near Gravesend. 
21. Nitzschia angularis 
22. Amphiprora alata 
23. Navicula Jennerii 
24. „ didyma 
25 punctulata 
26. Pinnularia cyprinus 
27. Stauroneis salina 
28. Pleurosigma angulatum 
29. hippocampus 
30. „ Balticum 
31. Doryphora amphiceros 
32. Achnantbes brevipes 
33. Grammatophora marina 
34. Podosira Montagnei 
35. Melosira nummuloides 
36. ,, sulcata 
37. „ salina 
38. Odontella aurita 
39. Orthosira marina 
Fresh-water Species from Gravesend. 
1. Cocconeis placentula 
2. Coscinodiscus minor 
3. Nitzschia sigmoidea 
4. Navicula cuspidata 
Marine and Brackish Water 
1. Amphora membranacea 
2. Coscinodiscus eccentricus 
3. ActinocycUis undulatus 
5. Navicula minutula 
6. Synedra ulna 
7. Cocconema cistula 
8. Cyclotella rotula 
from the' Thames near Hammersmith. 
4. Surirella Brightwellii 
5. Tryblionella gracilis 
6. acuminata 
