398 PBOCEEDINGS OP THE GEOLOGICAL SOCIETT. [May 10, 



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, Ole- 

 nellus, 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 

 earKest fossil-bearing rocks of Britain and Europe. 



The genus Anopolenus, very nearly allied to Paradoxides, Jiffords 

 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 ajjplies also to Erinnys and Holoceplialina, 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 

 " Eegio B." It is, however, by no means improbable that in this coun- 

 try 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, vs^hile 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 nn Olemis. 



