Chap. III.] LINGULA-FLAGS OF NOKTH WALES. 
43 
apparent, it has only occurred in such small quantities as to afford a very- 
thin testaceous covering to the few animals of this zone of early life. "We 
shall presently see that when the Silurian fauna became abundant, as in 
the next overlying strata, it was accompanied by a corresponding develop- 
ment of lime, which served as the material for the construction of the 
shells of the imbedded Mollusks. 
In ascending through the different stages of the Silurian system, it will 
also be seen that different species of the genus Lingula occurred most fre- 
quently at those intervals in which there -was a return to similar sedi- 
mentary conditions, i. e. whenever the muddy sea-bottom was only slightly 
impregnated with lime. 
With the Laurentian gneiss of Canada and other countries there are 
indeed those great thicknesses of crystalline limestone to which I have 
already referred. Upon the probable conditions under which the calcareous 
matter of these primeval rocks was deposited, partly through the medium 
of low animals like the Eozoon, and partly by chemical precipitation, I 
would refer the reader to Dr. Sterry Hunt's able memoirs on chemical geo- 
logy, in the * Eeports of the Geological Survey of Canada,' the ' Canadian 
Naturalist and Geologist,' the ' Geol. Soc. Journal,' and other works. 
Trilobites, or the earliest Crustaceans, abound infinitely more in the 
Silurian than in the next overlying system of rocks ; and the earliest which 
the labours of geologists have brought to light in Britain are Agnostus, 
Paradoxides, Conocoryphe, Olenus, Dikelocephalus, and Microdiscus. 
Fossils (5). Lingula-flags, North Wales. 
1. Lingula (Lingulella) Davisii, M'Coy. 4. Agnostus princeps, Salter ; 
2. Olenus micrurus, Salter. both of the natural shape and 
3. Cruziana semiplicata, Salter. distorted by cleavage. 
Besides these fossils, a small Crustacean of the Phyllopod tribe has been 
found — Hymenocaris vermicauda. A figure of this Crustacean is here given 
(Poss. 6. f. 1), from specimens in the collection of the Geological Survey. 
It is plentiful at Dolgelly and Tremadoc, in North Wales, associated with 
Lingulella Davisii. Tracks of Annelides also prevail, as in the rocks beneath 
and above the Lingula-flags ; sometimes they are of considerable size. 
In beds of the same age, near Bangor, have been also found two kinds 
of so-called Pucoids (p. 40), one of which is, perhaps, the Chondrites acu- 
tangulus of M'Coy ; the other is a species of the curious genus Cruziana 
(d'Orbigny), Poss. 5. fig. 3. 
