
EPART IL Secr. ii. § 2.] SILURIAN, | 675 
Appearing on the coast of Pembrokeshire at Marloes Bay, they range 
across South Wales until they are overlapped by the Old Red Sandstone, 
They emerge again in Carmarthenshire, and trend north-eastward as a 
narrow strip at the base of the Upper Silurian series, from a few feet to 
1000 feet or more in thickness, as far as the Longmynd, where, as a 
marked conglomerate wrapping round that ancient Cambrian ridge, 
they disappear. In the course of this long tract they pass suecessively 
and unconformably over Lower Llandovery, Caradoc, Llandeilo, and 
\ 
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SS 
SSS 
we 
2 
SS SVR 
SS RA 
PAIR 
SS 
SEN 

SSS 

Fie. 326.—Group oF PENTAMERI FRoM LLANDOVERY Rocks. 
a, Pentamerus oblongus (Sby.); 6, P. galeatus (Dalm.); ¢, P. Knightii (Sby.); d, P. ob- 
longus (Sby.); e, P. rotundus (Sby.) @); /, P. Knightii (small specimen); g, P. lin- 
guifer (Sby.); h, P. undatus (Sby.). 
Cambrian rocks. They consist of yellow and brown ferruginous sand- 
_ stones, often full of shells, which are apt to weather out and leave casts. 
Their lower parts are commonly conglomeratic, the pebbles being 
largely derived from older parts of the Silurian system. Here and 
_ there, where the organic remains become extraordinarily abundant, the 
strata pass into a kind of sandy limestone, known as the “ Pentamerus 
limestone,” from the numbers of this brachiopod contained in it. The 
fossils found in the May Hill Sandstones number 91 genera and 261 
species, of which only 136 species are confined to this group. 
: : g BR 
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