574 HISTORICAL GEOLOGY. 



into the Devonian. But in the western part in Wales, — and in the Lake 

 region to the north, — they lie unconformably beneath the Old Red Sand- 

 stone (Devonian), proving thereby that an epoch of local mountain-making 

 in that region closed the period. Similar evidence of disturbance exists in 

 Ireland. 



BIOLOGICAL PROGRESS. 



The records of the Upper Silurian era add to the terrestrial fauna the 

 earliest of Araclinids, or Spiders, in the form of Scorpions, and additional 

 species of Insects. The former are in the successional line of the Euryp- 

 terids, whose earliest species is Lower Silurian ; the Insects are structurally 

 in the line of the Myriapods, although no antecedent species of Myriapod is 

 yet known. The Cockroaches are Orthopters, and species of imperfect 

 metamorphosis, like the Hemipters. The relations of the above-mentioned 

 groups are illustrated in the course of the General Observations on the 

 Paleozoic, on pages 721, 722. 



Among marine Invertebrates, the era is marked by a large diminution in 

 the number of species and genera of Graptolites and Trilobites — Lower Silu- 

 rian characteristics; by an abundance of Cystoids and Orthids — also Lower 

 Silurian in aspect ; by an increase in the number and size of the Brachiopods 

 of the families of Spiriferids, Pentamerids, and Productids — Devonian char- 

 acteristics ; by an increase in the Pteropods of the Conularia type ; by an 

 increase in the number of Gastropods of the Platyceras (Capulus) type, and 

 in the number of species and genera of Polyp-corals, Crinoids, and Asterioids 

 — which also look toward the Devonian ; by an increase in the number and 

 variety of Eurypterids and Ceratiocarids — facts having the same bearing. 



Still more marked is the advance from Entomostracans to Tetradecapod 

 Crustaceans ; and far beyond this is the appearance of Insects. It is re- 

 markable that the first remains of Scorpions should have been found in 

 Europe and America in rocks of very nearly the same age. But it may be 

 that earlier specimens are yet to be found. 



Fishes, the only Vertebrates of the Upper Silurian, were represented by 

 Placoderms, the mail-clad type that first appeared in the Trenton Period of 

 the Lower Silurian, and possibly also by Selachians. But no remains of 

 other Ganoids have yet been found in the beds, although reported from the 

 Trenton. Rarity in fossils of lime-secreting aquatic species is not common. 

 Remains of Chimseroids, mostly cartilaginous species, also are absent. 



CLIMATE. 



There is no evidence that the climate of America was roughened by 

 frigid winds, or that the ocean was much modified in temperature by polar 

 c\irrents. The species living in the waters between the parallels of 30° and 

 45° were in part the same with, or closely related to, those that flourished 

 between the parallels of 65° and 80°. (See page 544.) From this life 

 thermometer we learn only of warm or temperate seas. 



