72 Roper, on the Diatomacece of the Thames. 
7. Mtzschia sigma 
8. Nitzschia parvtila 
9. Navicula elliptica 
10. Pinnularia directa 
11. Pleurosigma hippocampus 
12. Doryphora amphiceros 
13. Gomphonema marinnm 
14. Odontella aurita 
Fresh-water Species from the same Place. 
16. Pinnularia viridula 
17. „ stauroneiformis 
18. Pleurosigma attenuatum 
19. Synedra ulna 
20. Cocconema cymbiforme 
21. ,, cistula 
22. Gomphonema acuminatum 
23. constrictum 
24. Fragillaria virescens 
25. Diatoma vulgare 
26. Melosira arenaria 
27. „ varians 
28. Fragilaria capucina 
29. Coscinodiscus minor 
1. Epithemia turgida 
2. Cymbella Ehrenbergii 
3. Amphora ovalis 
4. Cocconeis placentula 
5. Campy lodiscus costatus 
6. Surirella biseriata 
7. Cymatopleura solea 
8. elliptica 
9. „ apiculata 
10. Nitzschia sigmoidea 
11. Navicula amphisboena 
12. „ crassinervia 
13. „ inflata 
14. cuspidata 
15. „ amphirhynchus 
From these lists it appears that at Gravesend, out of forty- 
seven species, eight only are decidedly peculiar to fresh water ; 
whilst at Hammersmith we find there are twenty-nine fresh- 
water species out of a total of forty-three ; showing, however, 
that the influence of the flood-tide, even at that distance from 
the sea, gives a decided character to the Diatomacece deposited 
by the water. The following ten species are all that are 
common to the three localities : — Coscinodiscus eccentricus, 
Actinocyclus undulatus, Tryblionella acuminata .,N it zschia sigma, 
Pleurosigma hippocampus, Doryphora amphiceros, Odontella 
aurita, Cocconeis placentula, Nitzschia sigmoidea, and Cocconema 
cistula, of which the three latter alone are peculiar to fresh 
water. These are all forms which more extended observation 
on the deposits of other river and estuary deposits will pro- 
bably prove to be most universal in their distribution. I have 
found most of them in the mud of the Avon from Bristol, and 
also in that deposited at Pembroke Harbour ; but it will re- 
quire a careful examination of many other deposits to prove 
that any have a purely local habitat, or are entirely confined 
to sea or fresh water. 
The following species which occur in the Thames have also 
been found by Professor Bailey in America, recorded in * Sil- 
liman's Journal of Sciences ' for 1845, vol. xlviii. p. 337 : — 
In the Mud from Charleston Harbour. 
Actinocyclus senarius 
Coscinodiscus eccentricus 
Eupodiscus argus 
Pinnularia didyma 
Pleurosigma Balticum 
Khaphoneis amphiceros 
rhombus 
Triceratium favus 
Zygoceros rhombus 
