CLASSIFICATION OF PALAEOZOIC ROCKS OF THE BRITISH ISLES. 303 
dark slates and shales, with contemporaneous beds of volcanic 
tuff. It has an average thickness, as now defined, of about 
2000 feet, and contains a distinct and important fauna. Most 
of the species are new, and amongst the trilobites is one genus 
{Placoparia) not elsewhere found in Britain. The fauna, which 
was worked out by me from the beds at Llanvirn, St. David’s, 
in 1874, is very rich in trilobites; and several genera appear 
here for the first time in the succession, as Illoenus, lllcenopsis , 
Barrcindea , Phacops , Placoparia , and Acidaspis. The fauna also 
contains many graptolites, chiefly of the genera Diplograptus 
and Didymograptus , and there are also large Cephalopods, 
Grasteropods, Brachiopods, and Lamellibranchs. Beds belong- 
ing to this group have been explored by Mr. Marr near 
Caernarvon ; they are probably to be found also near Shelve, in 
Shropshire. The lower portion of the so-called Llandeilo at 
Abereiddjr Bay belongs rather to this group than to the 
typical Llandeilo, and is here included in it. Didymograptus 
Murchisonii is supposed by Prof. Lapworth to be characteristic 
chiefly of these beds. 
-g- I Fine Black Slates interstratified with Beds of Tuff con- 
I taining Didymograptus Murchisonii , &c. 
Lower. Dark-grey Flags and Slates, with Placoparia , & c. 
Llandeilo group. — In the typical area (Llandeilo), Sir B. 
Murchison described the rocks belonging to this group in as- 
cending order, as dark schists and shales with sandstone courses, 
followed by calcareous grits and pebble beds, and succeeded 
by schists in which Asciphus tyr annus is conspicuous ; upon these 
occur calcareous, dark- coloured, finely laminated flagstones and 
schists. This group as it occurs in Pembrokeshire, after ex- 
cluding the lower beds which I propose now tp place in the 
Llanvirn group, may be divided into two parts ; The lower con- 
sisting of black calcareous shales and flags, followed by toler- 
ably compact limestone ; and the upper portion of black slates, 
flags, and flaggy sandstones. The beds are well exposed on the 
north shore of Abereiddy Bay, near St. David’s (see Sect. 2, p. 
308), and are there highly fossiliferous, Ogygia Bucliii , Calyrnene 
duplicata , Ckeirurus Sedgtvickii, Trinucleus fimbriatus, occurring 
in abundance. The Grlenkiln slates of Scotland are supposed 
by Lapworth to be the equivalents of the upper beds of this 
group. 
