40 
SILUKIA. 
[Chap. III. 
terized by other fossils, such as Conocoryphe invita (Foss. 8. fig. 1), C. 
depressa (fig. 2), Dictyonema sociale (fig. 3), together with a small Orthis 
and Olenus alatus, Beck, a Scandinavian species. 
These fossils occur at Penmorpha and Y-Wern near Tremadoc, and 
along the base of Moel-y-Gest; where a black slaty layer has been detected 
in this zone, with a surface covered by the Dictyonema above noted, 
indicating an undisturbed state of the former sea-bottom. Near Maentwrog 
and Ffestiniog the same layer has been traced, overlying the lighter- 
coloured and more arenaceous mass of the Lingula-flags ; and, both by its 
darker colour, and by exhibiting for the first time, in the series of strata, 
a Polyzoon and an Orthis, with the Agnostus pisiformis, this bed seems 
to be linked on to the lowest part of the Llandeilo Mags, into which it 
Fossils (8). Upper Lingula-flags, North Wales. 
1. Conocoryphe invita, Salter. 3. Dictyonema sociale, Salter; a Polyzoon allied 
2. Conocoryphe depressa, Salter. to Fenestella, and the oldest yet known of the group. 
graduates conformably, through shaly and schistose beds, charged with 
Lingulse, Trilobites, and other Silurian fossils. These schists and flagstones 
have been described on lithological grounds as ' Tremadoc Slates ' by Prof. 
Sedgwick; and subsequently Mr. Salter, obtaining from them a large 
series of organic remains, ranked them as two distinct formations, the 
Upper and Lower Tremadoc Slates. The distinction, however, of the latter 
from the Lingula-flags proper is difficult to be drawn ; and I still adhere 
to the classification put forth in the last edition of ' Siluria,' and regard 
the Upper Tremadoc slates as passing into and forming the lower part of 
the Llandeilo formation, because that band is charged with the Trilobites 
Asaphus and Ogygia, so characteristic of those strata. 
The minute subdivisions of the Lingula-flags and their associated slates, 
as worked out in North Wales, must be studied in the above-mentioned 
new volume of the Geological Survey (p. 244 &c), in which the valuable 
researches of Messrs. Homfray and Ash, in elucidating the fossils of this 
' Primordial Zone ' of Barrande, as also the recent successful labours of 
Mr. Hicks* in Pembrokeshire, are duly noticed by Mr. Salter. 
In South Wales, the Cambrian or Longmynd (lower?) rock at St. David's 
* See also Quart. Journ. G-eol. Soc. vol. xx. dial' fossils found at St. David's will be all enu- 
p. 233 ; vol. xxi. p. 476 ; Report Brit. Assoc. for merated in the Table, Appendix A. 
1865, p. 281 &c.; and above, p. 42. The 'primor- 
