486 
ELEMENTS OF GEOLOGY. 
Fiff. 573. 
Histioderma Hibernica^ Kin. Oldhamia beds. 
Head, Ireland. 
1. Showing opening of burrow, and tube with 
wrinklings or crossing ridges, probably pro¬ 
duced by a tentacled sea 'worm or annelid. 2. 
Lower and curved extremity of tube with five 
transverse lines. 
and Scotland. They have been called by Professor Sedg¬ 
wick the Longmynd or Bangor Group, comprising, first, the 
Harlech and Barmouth sandstones; and secondly, the Llan- 
beris slates. 
Harlech Grits. —The sandstones of this period attain in the 
Longmynd hills a thickness of no less than 6000 feet without 
any interposition of vol¬ 
canic matter; in some 
places in Merionethshire 
they are still thicker. 
Until recently these 
rocks possessed but a 
very scanty fauna. 
With the exception 
of five species of an- 
Bray nelids (see Fig. 460) 
brought to light by Mr. 
Salter in Shropshire, and 
Dr. Kinahan in Wick¬ 
low, and an obscure 
crustacean form, Palmopyge Pamsayi^ they were supposed to 
be barren of organic remains. Now, however, through the 
labors of Mr. Hicks,* they have yielded at St. David’s a rich 
fauna of trilobites, brachiopods, phyllopods, and pteropods, 
showing, together with other fossils, a by no means low 
state of organization at this early period. Already the 
fauna amounts to 20 species referred to 17 genera. 
A new genus of trilobite called Plutonia SedgioicJcii^ not 
yet figured and described, has been met with in the Harlech 
grits. It is comparable in size to the large Paradoxides Pa- 
vidis before mentioned, has well-developed eyes, and is cov¬ 
ered all over with tubercles. In the same strata occur other 
genera of trilobites, namely, Conocoryphe^ Paradoxides^ Mi- 
crodiscus^ and the Pteropod Theca (Fig. 568), all represented 
by species peculiar to the Harlech grits. The sands of this 
formation are often rippled, and were evidently left dry at 
low tides, so that the surface was dried by the sun and made 
to shrink and present sun-cracks. There are also distinct 
impressions of rain-drops on many surfaces, like those figured 
at p. 416. 
Llanberis Slates. —The slates of Llanberis and Penrhyn in 
Carnarvonshire, with their associated sandy strata, attain a 
great thickness, sometimes about 3000 feet. They are per¬ 
haps not more ancient than the Harlech and Barmouth beds 
last mentioned, for they may represent the deposits of fine 
* Brit. Assoc. Beport, 1868. 
