208 
SILUKIA. 
[Chap. IX. 
The Cup-corals chiefly belong* to the genus Petraia, which is eminently cha- 
racteristic of the Llandovery rocks, but comparatively rare either above or below 
them. The most common species is Petraia subduplicata, Foss. 15. f. 11, p. 90, 
which, with its elegant variety, crenulata, occurs in nearly every locality, both in 
the upper and lower portions of the deposit, in Wales, Scotland, and western 
Ireland. Petraia elongata, Phill., a larger species, is more common in the upper 
than in the lower beds, as at May Hill, Presteign, &c. ; while P. bina, Lons- 
dale, Foss. 53. f. 7, is apparently confined to the upper division, and ranges into 
the Wenlock rocks. P. sequisulcata of M'Coy, another large species, occurs 
near Girvan in Scotland, and, with P. subduplicata, M'Coy, is sometimes found 
in true Lower Silurian rocks. A square form, named Petraia quadrata by 
M'Coy, is found near Galway J it belongs, however, to the genus Goniophyllum, 
Milne-Edwards, and is the same as the Wenlock species of Gothland, the 
Goniophyllum quadratum, according to that author. There are also other 
species, as well as some Cup-corals of a larger size, which may be species of 
Cyathophyllum or Omphyma ; and these are chiefly in the upper strata of the 
formation. 
The class of Polyzoa, which is not absent from the Lower Silurian rocks, as 
before mentioned, is represented here by several species. Though not yet 
clearly defined, they have been chiefly referred to branched forms of the genus Pti- 
lodictya. Pt. dichotoma, p. 189, Foss. 31. f. 5, a Lower Silurian species, is the most 
frequent ; but the Wenlock forms, Pt. lanceolata andPt. scalpellum (Foss. 50. f. 6, 
& 51, pp. 216, 217), also occur, with several incrusting species. The most re- 
markable of these, if, indeed, it be allowed to remain in this group of fossils, is the 
Nidulites favus, p. 188, Foss. 30. f. 3. This fossil is characteristic of the Lower 
Llandovery series both in South Wales (Haverfordwest) and in the South of 
Scotland (Girvan), in each of which localities it is associated with the same 
fossils. It was for some time believed to be the nidus of a Gasteropod, similar 
to that made by the modern Natica *, the cup-like form of which it imitates. 
This explanation fails, however, when the fossil is more closely examined, since 
the cells are equal and regular on both sides of a central lamina and are set back 
to back like the cells of a honeycomb — whence the name. At the bottom of 
the cells a minute pit or depression is frequently visible, and this becomes a tu- 
bercle on the cast (in which state the fossils are always found). No trace of 
any upper plate or cover to the cells (which would exist in all the analogous 
types of Polyzoa) has been detected. The cells would seem to have been quite 
open at the top ; and as they are much larger than in any living species of 
that group, the exact affinity of the fossil has yet to be determined : possibly it 
belongs to the Amorphozoa. 
Fragments and stems of Encrinites are not uncommon ; but no perfect speci- 
mens have yet been obtained — although throughout the conglomerates and sand- 
stones of this age in South Wales Encrinite stems are the chief, and often the 
only fossils. These stems are usually hollow, a character common to all the 
Lower Silurian species ; while those of the Upper Silurian are mostly compact, 
with a small perforation only. The Llandovery rocks contain, however, both 
these kinds. A large species of Encrinite, with tuberculate joints alternating 
with very narrow plain joints, is found everywhere in the Upper Llandovery 
strata, and is figured from Presteign, where it abounds (PI. X. f. 1). 
Cystidea are very rare, a few detached plates occurring here and there in 
South Wales ; the curious genus Pleurocystites (p. 191), however, is found at 
* See Quart. Journ. Geol. Soc. vol. vii. p. 174. 
