The genus Paradoxides is represented in America by six species ; and these all appear to occur in the lower portion of the primordial zone. Of these species it seems probable that P. Thompsoni and P. vermontianus, which Mr. Billings has referred to a new genus, Olenellus, occur in a position higher than the forms from Massachusetts ; and as regards P. macrocephalus, Emmons, its position seems to be doubtful *. The Old World affords, including the new species alluded to in this communication as occurring in the purple and green rocks of St. David's, twenty forms ; and these are all very low down in the earliest fossil-bearing rocks of Britain- and Europe.
The genus Anopolenus, very nearly allied to Paradoxides, affords two species only ; and these have hitherto only been found in the dark- coloured rocks above the purple and green strata of St. David's ; and the same remark applies also to Erinnys and Holocephalina, a species of each of which occurs in the same rocks.
As regards the thickness of the strata extern to the British isles which afford the earlier forms of Trilobites, this, in Europe, contrasts very strongly with what our own country exhibits.
In Sweden, the strata representing the ancient fossiliferous rocks of St. David's are very thin, if we regard them as included in Angelin's " Regio B." It is, however, by no means improbable that in this country the underlying " fucoid sandstones " represent the lowest series of the fossiliferous rocks of South Wales.
In Bohemia, the primordial zone of Barrande, " Etage C," has a thickness of about 1200 feet.
The thickness of this zone in Spain seems to be very small. There are two bands representing the primordial rocks in that country. One has a thickness of about 40 metres, that of Sabero ; and the other seems to have a like thickness. They may, however, be the same band repeated by a fault.
In America the Potsdam sandstones, or their representatives, have very variable thickness. In some spots they are not more than 20 feet in depth, while in other localities they are said to attain to several thousand feet of thickness.
In Wales, if we assume the strata to which the term Lingula-flags has been applied, and those which in South Wales occur in a lower horizon, to represent the Potsdam sandstones, then we attain to a thickness much exceeding even the most extensive development of these sandstones, since the Lingula-flags would afford us about 6000 feet of strata where they are best developed, to which we have to add from 3000 to 4000 feet for the St. David's rocks in which fossils occur, as shown by the labours of Mr. Hicks.
List of Memoirs published since 1862 on the Rocks of the St. David's Promontory, and on their representatives in North Wales.
1. Salter, Quart. Journ. Geological Society, Feb. 1863.
2. Hicks, Trans, of the Geol. Soc. of Liverpool, Dec. 1863.
3. Salter, Quart. Journ. Geol. Soc. 1864.
- The form P. asaphoides, Emmons, seems to be an Olenus.
