470 
ELEMENTS OE GEOLOGY. 
common to the May-Hill group above, and 93 common to the 
rocks below. StricMandinia , {Pentamerus) levis^ which is 
common in the Lower Llandovery, becomes rare in the Up¬ 
per, while Feiitamerus oblongus (Fig. 546), which is the char¬ 
acteristic shell of the Upper Llandovery, occurs but seldom 
in the Lower. 
LOWER SILURIAN ROCKS. 
The Lower Silurian has been divided into, 1st, the Bala 
Group; 2d, the Llandeilo Flags; and, 3dly, the Arenig or 
Lower Llandeilo formation. 
Bala and Caradoc Beds. —The Caradoc sandstone was orig¬ 
inally so named by'Sir R. I: Murchison from the mountain 
called Caer Caradoc, in Shropshire; it consists of shelly sand¬ 
stones of great thickness, and sometimes containing much 
calcareous matter. The rock is frequently laden with the 
beautiful trilobite called by Murchison Trinudeus Caractaci 
(see Fig. 553, p. 472), which ranges from the base to the sum¬ 
mit of the formation, usually accompanied by Strophomena 
grandis (see Fig. 551), and Orthis vespertilio (Fig. 550), with 
many other fossils. 
Eig. 549. Fig. 550. Fig. 551. 
Orthis tricenaria^ 
Conrad. New 
York; Canada. 
^ nat. size. 
Orthis vespertilio, Sow. 
Shropshire, N. and 
S. Wales. One-half 
nat. size. 
Orthis {Strophomena) grandis, Sow. 
Two-thirds nat. size. Caradoc 
Beds, Horderley, Shropshire, 
and Coniston, Lancashire. 
Brachiopoda. —Nothing is more remarkable in these beds, 
and in the Silurian strata generally of all countries, than the 
preponderance of brachiopoda over other forms of mollusca. 
Their proportional numbers can by no means be explained 
by supposing them to have inhabited seas of great depth, for 
the contrast between the palaeozoic and the present state of 
things has not been essentially altered by the late discoveries 
made in our deep-sea dredgings. We find the living brachi¬ 
opoda so rare as to form about one forty-fourth of the whole 
bivalve fauna, whereas in the Lower Silurian rocks of which 
we are now about to treat, and where the brachiopoda reach 
their maximum, they are represented by more than twice as 
many species as the Lamellibranchiate bivalves. 
