542 
SILUEIA. 
F. — Astacodermata. 
Mr. Henry Woodward has supplied the following note on the peculiar little 
fossils referred to at pages 133 & 134 : — 
With the exception of Astacoderma serratum (which is perhaps a fragment of 
the margin of a carapace or of a body-segment of Ceratiocaris), and of A. spi- 
nosum (not clearly referable to Ceratiocaris), the several forms of Astacoderma 
described and figured by Dr. Harley are doubtless founded on the teeth of 
Phyllopod Crustacea, such as Ceratiocaris. For examples of such teeth in 
Dictyocaris, an allied genus, see the ' Geological Magazine/ vol. ii. p. 401, pi. 11. 
G. — Old Red Sandstone of Forfarshire (p. 250). 
In treating of the classification and palaeontology of the Old Eed Sandstone 
of Forfarshire and other parts of Scotland at p. 250, allusion to the researches of 
the Rev. Hugh Mitchell was inadvertently omitted. In the Quart. Journ. Geol. 
Soc. vol. xvii. p. 145 et seq., Mr. Mitchell describes the position of the beds of the 
Old Eed Sandstone in the counties of Forfar and Kincardine, and at p. 146 he 
classifies the fossils that have been found there and elsewhere ; and amongst 
them he enumerates the small spores known as Pachytheca, which are not rare 
in the uppermost Silurian beds at Ludlow in Shropshire. 
In addition to the references made at p. 251 &c. to Mr. Powrie's discoveries, 
it is right to refer the reader to the publications of this author on the Old Eed 
Sandstone and its fossils, in the Quart. Journ. Geol. Soc. vol. xvii. p. 534 et seq., 
and vol. xx. p. 413 et seq. 
H. — Caithness Flagstones of the Old Red Sandstone (p. 258). 
The Flagstones of Caithness, which were first described by me in the year 
1827, under the name of ' Bituminous schists' (Trans. Geol. Soc. 2nd ser. vol. ii. 
p. 213), are in many places impregnated with bitumen, chiefly resulting from 
the vast quantity of fishes imbedded in them. Their most durable and best qua- 
lities as flagstones are derived from an admixture of this bitumen with finely 
laminated siliceous, calcareous, and argillaceous particles, the whole forming a 
natural cement more impervious to moisture than any stone with which I am 
acquainted. 
The following analyses of several specimens of Caithness flagstone and the 
accompanying bituminous shale, from the celebrated quarries of Castle Hill, 
belonging to my friend Mr. G. Traill, M.P., are indeed of high value, having been 
prepared under the superintendence of that distinguished chemist Dr. Hofmann. 
These results completely sustain the opinion I was led to form upon the spot, 
that the peculiar tenacity and durability of these flagstones is due to the manner 
in which the silica and alumina are cemented together by certain proportions of 
calcareous and bituminous (organic) matter. 
Mineral analyzed. 
Silica and 
Silicate 8 
insoluble 
in H CI. 
Oxide of 
Ii'on and 
Alumina. 
Carbonate 
of Lime. 
Organic 
matter. 
Water, 
loss at 
100° C. 
Salts of 
Magnesia, 
the Al- 
kalies, &c. 
Total. 
No. 16. Top flag . 
No. 7. Middle flag 
Bituminous Shale. 
No. 1 . Bottom flag 
68- 40 
69- 45 
69-96 
61-39 
10- 21 
11- 50 
8-15 
4-87 
10-93 
10-66 
7-72 
21-91 
3-88 
5-79 
10-73 
3-40 
0-42 
0-40 
0-53 
0-20 
6-16 
2-20 
2-91 
8-23 
10000 
10000 
10000 
100-00 
