Xvlll DESCRIPTIVE REMARKS. 
Fosstts oF THE LLANDEILO FLAgs. 
The strata next in order of superposition are what are called the Llan- 
deilo Flags, a series of deposits consisting of black earthy slates, and 
interstratified sandy flagstones, with occasional calcareous bands, which 
sometimes become regular limestones. Some of these beds contain a 
great abundance of Fossils, particularly in the neighbourhood of the 
town from which it derives its name—Llandeilo Fawr, in Caermarthen- 
shire; and near Builth, in Brecknockshire ; similar rocks occupying parts 
of both North and South Wales. This Formation also occurs in the Si- 
lurian region on the western flank of the Stiper stones of Shropshire, as 
well as in Cumberland; the Skiddaw slate, its representative, forming 
the lowest band in that slaty region.* Fossils indicating strata belonging 
to this Formation have also been collected at various places in the south 
and central parts of Ireland, and probably also in the north: there is, 
however, from the absence of continuous sections, much difficulty in 
determining their relative positions at present. The advancement of the 
Geological Survey, combined with a more extended examination of the 
Fossils, will doubtless afford a better clue as to the equivalents of the 
various members of the Silurian Formation in that country. 
This group of strata has been separated into two series— Upper and 
Lower—and the Fossils enumerated in the Appendix to the Geology of 
North Wales,} as well as in the Appendix to Siluria, third edition. 
The fossils of the Llandeilo Formation are more varied as to the 
classes represented, as well as more numerous, than in any of the pre- 
ceding Formations ; they consist of afew Corals, very few Crinoidal re- 
mains; and a peculiar group of organisms, called Graptolites—which 
first make their appearance in the lower division, attaining their maxi- 
mum development in the upper series of these strata. As these remark- 
able Fossils are entirely confined to the Silurian Rocks, they become 
very important indicators of Lower Paleozoic strata. Of Molluscous 
shells, Brachiopods are the most numerous, particularly the Orthide 
and Lingule; we have in these strata the earliest evidence of Lamelli- 
branch bivalves. The Gasteropoda, or univalves, were mostly of the 
Kuomphaloid type; both these classes, most abundant at the present 
day, having been but poorly represented in these ancient strata. On 
the contrary, shells referred to Nucleobranchiata, as Heteropoda, the 
class Pteropoda, and the now almost extinct class of Cephalopods, were 
the prevailing forms. 
The group of Trilobites existed in great numbers during this epoch, 
several new genera first making their appearance, such as Adglina, 
Trinucleus, Ampyx, and Calymene. 
The Fossils figured on Plate vi. are of a miscellaneous character. 
Paleochorda mapor, fig. 1, is believed to be a plant belonging to the sea- 
weeds or Algze ; Pyritonema fasciculus, fig. 3, Protovirgularia dichotoma, 
* Introduction to Brit. Sil. Brachiopoda, by Sir R. I. Murchison, Pal. Soe. p. 25. 
+ Memoirs of the Geological Survey, vol. iii., pp. 256, 258. 
