280 PEOCEEErisrGs of the geological society. 



New York by an American professor, and another species by Pro- 

 fessor Desor of Neuchatel. The other genus, found and described 

 by Dale Owen, is from the Upper Mississipi and its tributary the 

 River Lacroix. Some genera, as Phacops and Calymene are con- 

 tinued through the Silurian and Devonian systems, and without 

 change of form in some. More than half of the Trilobites of New 

 York and Wales are of Lower Silurian origin ; and twelve genera 

 never leave it. A quarter of the whole meet in Trenton Limestone, 

 where they attain their maximum, as in upper Bala, its Welsh re- 

 presentative ; and then they gradually decrease in numbers upwards 

 and downwards, not one appearing in the Calciferous sandstone. 

 The genera Olenus and Beyrichia, which in Europe are among the 

 very first animals visible, in New York delay their appearance 

 until the Mid-silurian stage is partly deposited. The singular 

 Crustacean, Eurypterus remipes, appears in both countries nearly 

 at the same epoch. In Wales the Crustacea occupy every sedi- 

 mentary group, and by 210 appearances (Salter); in New York 

 they show themselves in 10 sections, by 68 appearances. In this 

 last area, only 9 species appear posterior to the deposition of the 

 Niagara strata (including the Devonian), which is a decrement as 

 rapid as that observed by Murchison in Russia*. The case is 

 much the same in Wales. In the '' Eastern Region" of Phillips, 

 however (embracing Usk, Tortworth, Mayhill, Woolhope, &c.), the 

 Upper Silurian Trilobites are 19, to 8 Lower Silurian, — a con- 

 dition of things very different from that seen in other parts of 

 Wales. The two basins of which we always speak possess 18 genera 

 in common ; but New York has 5, and Wales 21, which are distinct, 

 and possibly peculiar. In both hemispheres, Acidaspis, Agnostus, 

 Bronteus, Ogygia, and Proetus are non-recurrent, — a remarkable 

 coincidence. In the number of species neither basin, as yet, ap- 

 proaches that of Bohemia (230 species of Trilobites), and Scandinavia 

 (350 species). The Irish Trilobites assimilate to those of New York 

 in their common genera, Isotelus and Asaphus. Few genera originate 

 in Upper Silurian, — some of the successive appearances there being 

 recurrent forms. The Welsh basin is twice as prolific as the Ameri- 

 can ; but the animals appear in both at the same period — Llandeilo 

 or Trenton (which last includes the Chazy and Birdseye limestones). 

 Precedence does not seem to depend on physiological grounds. The 

 genera rich in species in the one country are so in the other, and 

 vice versa, but with exceptions. 



2. Calciferous Sandstone. — In this set of strata no members of the 

 Potsdam fauna exist. Six Gasteropoda, with the beautiful Ophileta, 

 three Orthoceratites, a BeUerophon, and a Eaomphalus are substituted. 

 Individuals are plentiful, but species are rather few. This appHes 

 to the genus Eaomphalus, which is first met with in this rock, which 

 is equably spread over Welsh Siluria, where its species commence 

 their existence almost on the same horizon with their American con- 

 geners. The species of Orthoceratites alluded to live and die here ; 

 but the genus survives through 9 sections in New York, as carnivo- 

 * Geol. of Russia, vol. ii. p. xxii. 



