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SEPTEMBER 25, 1913] 
Girvanella nodules occar conspicuously on the 
weathered surfaces, being so abundant as to con- 
stitute thick layers of limestone. 
Solenopora compacta, var. Peachii, which, likewise, 
forms important masses of limestone, occurs, like 
Girvanella problematica, in the Girvan area, but at a | 
somewhat lower horizon, namely, in the nodular 
limestone and shales forming the lower sub-division of 
the Stinchar limestone. It was originally described 
from pebbles in the Old Red Conglomerate of 
Habbie’s Howe by Nicholson and Etheridge, jun., 
under the name Tetradium Peachii, and was subse- 
quently discovered to occur plentifully in the Ordo- 
vician limestone at Tramitchell and Craighead, and to 
be synonymous with Solenopora compacta (Bill). In 
the shales associated with the nodular limestone of 
Craighead it occurs as spheroidal and _botryoidal 
nodules up to 14 in. in diameter; while in the lime- 
stone itself the nodules may have a diameter of 3 in. 
On freshly fractured surfaces it appears as_ buff- 
coloured on brownish spots, having a compact por- 
cellanous texture, while weathered surfaces often show 
a concentric structure. Under the microscope the 
tubes of this species vary in diameter from 50—8o u. 
In the Geological Survey Memoir it is also recorded, 
under the original name of Tetradium Peachii, from 
the Stinchar limestone of Benan Burn, Millenderdale, 
and Bougang, where it is accompanied by two other 
species, S. filiformis and S. fusiformis,’ which con- 
tributes conspicuously to the deposit, often forming 
large masses of limestone. The horizon of the 
Stinchar limestone is correlated by Prof. Lapworth 
with the Craighead limestone, and considered to repre- 
sent the summit of the Llandeilo or the base of the 
Caradoc of the Shropshire district. It is of interest to 
note that Solenopora is here accompanied at times by 
well-marked oolitic structure, and that the same is 
true of the pebbles with which it is associated in the 
conglomerate at Habbie’s Howe. 
Although the marked development of Solenopora 
found in the Stinchar limestone ceases'with the advent 
of the Benan conglomerate, the genus appears to have 
survived in the district into Upper Caradoc times, for 
Dr. Brown describes a new _ species (S._ litho- 
thamnioides) from Nicholson’s collection from the 
Ordovician (? Silurian) at Shalloch Mill, where it 
occurs in conical masses the size of a walnut. The 
only beds in which we might expect alge to occur 
in this locality are the nodular limestones or Dionide 
beds of the Whitehouse group of Prof. Lapworth’s 
classification, but there is no mention of Tetradium or 
Solenopora from this locality in the fossil lists cited 
from Mrs. Gray’s collection in the survey memoir. 
As this point is of some interest, I have consulted 
Mrs. Gray, who very kindly sent me some small 
nodules which she had collected from the Whitehouse 
beds of Shalloch Mill. On slicing one of these I find 
that it is undoubtedly a Solenopora, and probably the 
species figured by Dr. Brown as S. lithothamnioides. 
A tangential section cut from this specimen shows 
clearly why the original specimen of Solenopora from 
Craighead was mistaken for Tetradium by Nicholson 
and Etheridge, jun. 
South of the Scottish border there is, so far as I 
am aware, only one locality from which calcareous 
algze have been recorded in rocks of Ordovician age, 
namely, Hoar Edge in Shropshire. Here large 
examples of Solenopora compacta were obtained in 
1888 by Prof. Lapworth from the caleareous layers 
near the base of the Hoar Edge sandstone. The 
specimens were handed to Prof. Nicholson, who 
records the circumstance in his description of S. com- 
pacta in the Geological Magazine for 1888. The form 
J4 Brown, of. crt.,pp. 195-197. 
NO. 2291, VOL. 92] 
NATURE 
115 
occurs here at the base of the Caradoc beds, and there- 
fore at a horizon which corresponds closely to that of 
the Craighead limestone of Girvan. 
Prof. Lapworth also informs me that he has ob- 
tained specimens of Solenopora from a limestone in 
south-west Radnorshire. As the upper portion of the 
limestone in which it is found contains a Silurian 
fauna, it is possible that it is here present at a higher 
horizon, though the constancy with which it occurs 
elsewhere, in beds of Llandilo-Caradoc age, would 
seem to point to the possible presence of beds of 
Upper Ordovician age in this area. In any case, its 
occurrence here is of considerable interest. 
Foreign Ordovician. 
Outside of Britain, one of the most interesting de- 
velopments of calcareous algae in rocks of Ordovician 
age occurs in the Baltic provinces. 
As already stated, Solenopora was first recorded 
from Herrkiill in Esthonia, by Dybowski under the 
name of Solenopora spongioides. It occurs here 
in the Upper Caradoc or Borckholm beds of Schmidi’s 
classification—where it makes up thick beds of lime- 
stone—and it is noteworthy that this horizon is prac- 
tically identical with that at which S. lithotham- 
nioides (Brown) occurs at Shalloch Mill. 
Other specimens of Solenopora were collected by 
Prof. Nicholson in Saak, south of Reval, from the 
underlying Jewe beds, an horizon which corresponds 
very closely to that of the Craighead limestone of 
Girvan. Speaking of these beds, Nicholson and 
Etheridge remark: ‘At this locality S. compacta not 
only occurs as detached specimens of all sizes, but it 
also makes up almost entire beds of limestone ; indeed, 
some of the bands of limestone at Saak look like 
amygdaloidal lavas, while others have a_ cellular 
appearance from the dissolution out of them of the 
little pea-like skeletons of this fossil.” 
In Prof. Nicholson’s collection from these beds Dr. 
Brown afterwards distinguished two new species, 
namely, S. nigra and S. dendriformis. Thus in the 
Ordovician rocks of Esthonia, Solenopora plays quite 
as important a part (as a rock-forming organism) as 
it does in the Girvan district in Ayrshire. 
In Norway again, in the Mjosen district to the north 
of Christiania, Solenopora occurs  plentifully in 
Stage 5 of Kiaer's Ordovician series. Here it is 
very abundant and often builds entire beds, while, 
further east, at Furnberg, Kiaer again records the 
occurrence of abundant nodules of Solenopora com- 
pacta, var. Peachii. 
In addition to Solenopora, however, examples of 
another important group of calcareous alge, the 
Siphon, occur in great abundance in the Ordovician 
rocks of the Baltic region, where they play a part 
in the formation of calcareous rocks, scarcely less 
| important than that played by Gyroporella and Diplo- 
pora in the rocks of the Alpine Trias. 
The chief forms belong to the 
Dasycladaceze, represented by the 
Neomeris, and include the genera Paleoporella, 
Dasyporella, Rhabdoporella, Verimporella, Cyclo- 
crinus, and Apidium. These algal limestores repre- 
sent the beds from the Jewe to the Boreckholm beds 
inclusive. They were originally investigated by Dr. 
E. Stolley, who described their occurrence in_ the 
numerous boulders which are strewn over the North 
German plain in Schleswig-Holstein, Pommerania, 
Mecklenburg, and Mark-Brandenberg. Many of these 
boulders can be identified by their lithological char- 
acter and fossil contents as belonging to the Jewe 
beds of the Baltic Ordovician formations. Others 
have been derived from the overlying Wesenberg 
limestones, while yet others occur which resemble the 
family of the 
recent genus 
