484 
ELEMENTS OF GEOLOGY. 
Lingula Flags. —Next below the Tremadoc slates in North 
Wales lie micaceous flagstones and slates, in which, in 1846, 
Mr. E. Davis discovered the Lingula {Lingulella)^Yig, 570, 
named after him, and from which was derived the name of 
Lingula flags. These beds, which are palaeontologically the 
equivalents of Barrande’s primordial zone, are represented 
by more than 5000 feet of strata, and have been studied 
chiefly in the neighborhood of Dolgelly, Ffestiniog, and Port- 
madoc in North Wales, and at St. David’s in South Wales. 
They have yielded about forty species of fossils, of which 
six only are common to the overlying Tremadoc rocks, but 
Fig. 569. Fig. 570. Fig. 571. 
Lingula Flags” of Dolgelly, and Ffestiniog; N. Wales. 
Hymenocaris vermicauda, Salter. Lingulella Davisii, M‘Coy. Olenus micruruSy 
A Phyllopod Crustacean. One- a. One-half natural size. Salter. One-half 
half natural size. 5 . Distorted by cleavage. natural size. 
the two formations are closely allied by having several char¬ 
acteristic “ primordial ” genera in common. Likelocephalus^ 
Olenus (Fig. 571), and Conocoryphe are prominent forms, as 
is also Hymenocaris (Fig. 569), a genus of phyllopod crus¬ 
tacean entirely confined to the Lingula Flags. According 
to Mr. Belt, who has devoted much attention to these beds, 
there are already palaeontological data for subdividing the 
Lingula Flags into three sections.* 
In Merionethshire, according to Professor Ramsay, the Lin¬ 
gula Flags attain their greatest development; in Carnarvon¬ 
shire they thin out so as to have lost two-thirds of their 
•thickness in eleven miles, while in Anglesea and on the Menai 
Straits both they and the Tremadoc beds are entirely absent, 
and the Lower Silurian rests directly on Lower Cambrian 
strata. 
LOWER CAMBRIAN. 
Menevian Beds. —Immediately beneath the Lingula Flags 
there occurs a series of dark gray and black flags and slates 
alternating at the upper part with some beds of sandstone, 
the whole reaching a thickness of from 500 to 600 feet. 
These beds were formerly classed, on purely lithological 
* Geol. Mag., vol. iv. 
