194 



PALEOZOIC TIME — LOWER SILURIAN. 





of Lingula flags to some of the beds. Trilobites are the prominent 

 life of the period. 



Characteristic Species. 



Sea-weeds. — Besides the Fucoids, there are two species of Oldhamia 

 found in the Cambrian rocks of Ireland, — fig. 256 A, Oldhamia antiqua ; 257 A, 

 0. radiata. They were supposed by Forbes to be Bryozoan, but are generally 

 regarded as Sea-weeds or Corallines. 



Figs. 256 A-262 A. 

 61, 



Fig. 256 A, Oldhamia antiqua ; 257 A, 0. radiata ; 258 A, Lingula Davisii ; 259 A, Agnostus 

 Rex; 260 A, Olerms micrurus; 261 A, Sao hirsuta (X %)l 262 A, Hymenocaris vermi- 

 cauda (X %)• 



Radiates. — Of Polyps, none ; of Acalephs, Graptolites ; of Echinoderms, a 

 few species of Crinoids of the family of Cystids; occurring in Bohemia. 



Mollusks. — Brachiopods. — A few species of the Lingula and Orihis family. 

 Fig. 258 A, Lingula Davisii of the Lingula flags of North Wales. Conchifers. — 

 One species is reported as occurring in Great Britain. Pteropods. — There are a 

 number of species of Theca in Bohemia, Sweden, and England. 



Articulates. — Trilobites are more numerous in species than any other 

 group. Over seventy have been found in Scandinavia, and nearly thirty in Bo- 

 hemia. The common genera are Paradoxides, Agnostus, Olenus, Conocephalus, 

 Ellipsocephalus, and Sao. Fig. 259 A, Agnostus Rex, from Skrey, in Bohemia; 

 fig. 260 A, Olenus micrurus Salter, from the Lingula flags of North Wales ; fig. 

 261 A, Sao hirsuta, of Bohemia. 



Phyllopods appear first in this period, occurring fossil in the Primordial rocks 

 of Great Britain. Fig. 262 A is the Hymenocaris vermicauda, from North Wales. 

 It is like a shrimp in form, but has no projecting movable eyes. Moreover, in 

 place of ordinary legs, if a true Phyllopod, it had foliaceous appendages, which 

 were too delicate to be preserved (whence the name, from tyvWov, leaf, and novg, 

 foot). The existence of such Crustaceans at the present day, and the absence 

 in the fossils of feet and of pedicellate eyes, are evidence that the species are 

 of this type. 



III. Igneous Action and Disturbances. 



Through New York and the greater part of the West no evidences 

 of disturbance have been observed that can be traced to the 

 Potsdam period. The rocks are for the most part nearly horizontal 



