Part Il. Sect. i.§ 2.) CAMBRIAN. 655 
found in the Longmynd group, and some annelide-tracks likewise occur. 
The mollusca are represented by six species of brachiopods of the genera 
Discina, Lingulella, Obolella, and Orthis; 6 pteropods (Cyrtotheca, Theca) 
have been met with. The earliest entomostracan (Hntomis) and cystidean 
(Protocystites) yet discovered occur in the Menevian fauna. 
-8. Lingula Flags.—These strata, consisting of bluish and black slates 
and flags, with bands of grey flags and sandstones, attain in some parts 
of Wales a thickness of more than 5000 feet. They received their name 
from the discovery by Mr. EH. Davis (1846) of vast numbers of a Lingula 
(Lingulella Davisit) in some of their layers. They rest conformably upon, 
and pass down into, the Menevian beds below them, and likewise graduate 
into the Tremadoc group above. They are distinguished by a 
characteristic suite (71 species) of organic remains. The trilobites 
include the genera Agnostus, Anopolenus, Conocoryphe, Dikelocephalus, 
- Erinnys, Olenus, and Paradowides. ‘The earliest phyllopods (Hymenocaris) 
and heteropods (Bellerophon) occur in these beds. The brachiopods 
include species of Lingulella (L. Davisii), Discina, Obolella, and Orthis. 
The pteropods are represented by three species of Theca. Several 
annelides (Cruziana) and polyzoa (Fenestella) likewise occur. 
According to Mr. Etheridge, the Lingula flags may be grouped into 
three zones, each characterized by a peculiar assemblage of organic 
-yemains. ‘The lower division contains 36 species, of which seven are 
peculiar to it. The middle zone, which is of quite subordinate value, 
has yielded five species, two of which (Conocoryphe bucephala and Lingu- 
lella Davisii) pass down into the lower division, one (Kutorgina cingulata) 
ascends into the upper, and two (Lingula squamosa and Bellerophon cam- 
brensis) are peculiar. ‘I'he upper zone has yielded 41 species. Of these 
ten pass up into the Tremadoc beds, while two (Lingulella lepis and 
L. Davisii) continue on into the Arenig group.” 
4, Tremadoc Slates—This name was given by Sedgwick to a group of 
dark grey slates, about 1000 feet thick, found near Tremadoc in 
Carnarvonshire, and traceable thence to Dolgelly. Their importance as 
a geological formation was not recognized until the discovery in them of 
a remarkably abundant and varied fauna which now numbers 84 species. 
They contain the earliest crinoids, star-fishes, lamellibranchs, and 
cephalopods yet found. The trilobites embrace some genera (Agnostus, 
Conocoryphe, Olenus, &c.) found in the Lingula flags, but include also 
the new forms, Angelina, Asaphus, Cheirurus, Neseuretus, Niobe, Ogygia, 
Psilocephalus, &c. The same genera, and in some cases species, of 
brachiopods appear which occur in the Lingula flags, Orthis Carausti and 
Lingulella Davisit being common forms. Dr. Hicks has described 12 
species of lamellibranchs from the T'remadoc beds of Ramsay Island and 
St. David’s, belonging to the genera Ctenodonta, Palxarca, Glyptarca, 
Davidia, and Modiolopsis. The cephalopods are represented by Orthoceras 
sericeum and Oyrtoceras preecox ; the pteropods by Theca Davidii, T. oper- 
culata, and Conularia Homfrayt ; the echinoderms by a beautiful star-fish 
(Palzasterina ramseyensis) and by a crinoid (Dendrocrinus cambrensis).? 
Careful analysis of the fossils yielded by the Tremadoc beds suggests 
a division of this group into two zones. According to Mr. Etheridge, 
the Lower Tremadoc rocks have yielded in all 28 genera and 58 species, 
1 Etheridge, Q. J. Geol. Soc, 1881. President’s address, p. 48. 
2 Hicks, Quart. Journ. Geol. Soc. xxix. p. 39. 
