408 
SILURIA. 
[Chap. XVII. 
15111 
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sandstones with Bilobites and 
Scolithus as equivalent to the 
same beds (Bull. Soc. Geol. 
ser. 2. vol. xx. p. 126). It may, 
however, be suggested that 
these beds rather represent the 
upper portion of the Primor- 
dial Zone, containing the same 
fossils as the quartzose rocks of 
the Stiper Stones, which, like 
those of France, directly under- 
lie the Llandeilo formation. 
The great schistose and slaty 
mass which follows (Ardoises 
de Parennes, No. 7 of the dia- 
gram) forms the inferior limit 
of Silurian life as known in 
France. From its wide exten- 
sion and its fossils, this forma- 
tion (the Angers Slates) merits 
great attention ; for it is doubt- 
less, as M. de Verneuil states, 
the representative of the Llan- 
deilo rocks of Britain, of Bar- 
rande's ' Second Zone ' in Bo- 
hemia, and of the Orthocera- 
tite-limestones of Scandinavia, 
Russia, and North America. 
It is, in fact, the dominant 
member of the rocks termed 
Lower Silurian by all the au- 
thors who have described them 
in those countries. 
The slaty masses of Brittany 
are characterized, like their 
equivalents elsewhere, by a 
profusion of Trilobites. Un- 
fortunately these Crustaceans 
are, for the most part, much 
flattened and distorted; but 
the following species have been 
recognized ; and some of them 
were long ago figured by Guet- 
tard and Brongniart * : — 
Calymene Tristani, Brong. ; 
C. Arago, Rouault ; C. Verneuili, 
Rouault ; Asaphus G-uettardi, 
Brong. ; Ogygia Desmaresti, 
Brong. ; Illaenus giganteus, 
Burm. (the last two are gigantic 
species) ; 111. Beaumonti, Rou- 
* Crustacea Fossiles : Paris, 1822. 
