460 TRANSACTIONS OF SECTION ©. 
originally described by Nicholson and Etheridge, jun., under the name 7'etradium 
Peachit, from pebbles of the Old Red Conglomerate of Habbie’s Howe, and was 
afterwards discovered to occur plentifully in the Ordovician limestone at 
Tramitchell and Craighead. It was subsequently found by Nicholson to be 
synonymous with the form Solenopora (Stromatopora) compacta (Bill). At 
Craighead it occurs in the shales as spheroidal and botryoidal nodules up to 
4 cm. in diameter, while in the limestone itself the nodules may have a diameter 
of 75cm. On freshly fractured surfaces it appears as buff-coloured on brownish 
spots, having a compact porcellaneous texture, while weathered surfaces often 
show a concentric structure. Under the microscope the tubes of this species 
vary in diameter from 50-804. 
In the Geological Survey Memoir it is 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, 8. filiformis and 
S. fusiformis,”= which contribute conspicuously to the deposit, often forming 
large masses of limestone. The horizon of the Stinchar Limestone is correlated 
by Professor Lapworth with the Craighead Limestone, and considered to repre- 
sent the summit of the Llandeilo or the base of the Caradoc rocks of the Shrop- 
shire 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 Lime- 
stone ceases with the advent of the Benan conglomerate, the genus appears to 
have survived in the Girvan district into Upper Caradoc times, for Dr. Brown 
describes a new species (S. lithothamnioides) from Nicholson’s collection from the 
Ordovician (? Silurian) beds at Shalloch Mill, where it is said to occur 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 White- 
house Group of Professor Lapworth’s classification, but there is no mention of 
T'etradium 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 
kindJy 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. lithotham- 
nioides. A tangential section cut from this specimen shows clearly how the 
original specimen of Solenopora from Craighead was originally confused with 
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 Algz have been recorded in rocks of Ordovician 
age, namely, Hoar Edge in Shropshire. Here large examples of Solenopora 
compacta were obtained in 1888 by Professor Lapworth from the calcareous 
layers near the base of the Hoar Edge Sandstone. The specimens were handed 
to Professor Nicholson, who records the circumstance in his description of 
S. compacta in 1888.24 The form occurs here at the base of the Caradoc beds, 
and therefore at an horizon which corresponds closely to that of the Craighead 
Limestone of Girvan. 
Professor Lapworth also informs me that he has obtained 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 Llandeilo-Caradoc age, would seem to point to 
the possibility that beds of Upper Ordovician age are also present in. this area. 
In any case, its occurrence here is of considerable interest. 
Foreign Ordovician. 
Outside of Britain, the most important development of Calcareous Alge in 
rocks of Ordovician age occurs in the Baltic Provinces. 
As already stated Solenopora was first recorded from Herrkill in Esthonia, 
by) Dybowski under the name of Solenopora spongioides. It occurs here in 
23 Brown, op. cit. (*), pp. 195-197. 24 Op. cit. (4), p. 22. 
