Baily— Cambrian Rocks and Fossils. 391 
rocks containing these fossils accessible at low water are what are 
called the Periwinkle Rocks, consisting of a greenish fine-grained 
grit with irregular joints crossing the numerous layers of deposit. On 
striking off a piece of this rock, it will be seen to exhibit a wrinkled 
appearance; and, on closer inspection, this will be found to have 
been caused by numerous irregular star-like markings radiating from 
a centre, which preserve a certain amount of uniformity, and appear 
to permeate the whole mass of rock, every one of the numerous 
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Fie. 3. 
Oldhamia antiqua (a) with burrows of Annelids, Arenicolites didyma (6), and surface-tracks (c). 
CAMBRIAN GREEN SHALES, BRAY HEAD, COUNTY OF WICKLOW. 
lamine, where exposed, exhibiting a surface covered by these pecu- 
liar little fossils. The above wood-engraving (fig. 3) gives a repre- 
sentation of the appearance presented by a fragment of rock full of 
these characteristic fossils. This, the most abundant species, which 
is found at several other stations in these cliffs, has been named 
Oldhamia radiata. 
A second species, called Oldhamia antiqua, occurring, like the 
former, in both green and purple shales and grits, has been also met 
with at various places along the shore, as well as in the rocks ex- 
posed on the side of the hill; but the two kinds seldom if ever 
occur together in the same bed. The latter species, consisting of 
