T8 Dynamic Theory. 



the animal. In length, they varied from a few inches to two feet. 

 There were, first and last, nearly 1 , GOO species of them. They became 

 extinct in the beginning of the Carboniferous age. They belong to the 

 lower division of the crustaceans, viz., the Enlomostracans. They are 

 ver} r like the larvae of the King crabs and other crustaceans, and are, 

 very probably, also the antecedent relatives of the Eurypterus, an ad- 

 vanced entomostracan, which came in during the last period of the Silu- 

 rian age. 



The life of the upper part of the Lower Silurian, requires no special 

 mention. 



The shales of the Hudson period contain a great amount of carbon- 

 aceous material, which was quite certainly derived originally from the 

 plants, although the animal remains, doubtless, directly contributed to 

 them. This is evidence of the growing amount of organic matter on 

 the earth both animal and vegetable. The rocks, too, attest many 

 variations in minor details of animal development. Species and genera 

 are constantly becoming extinct, and new ones are being introduced, 

 yet the general types remain more or less constant. The new species 

 are, as a rule, more complicated in structure the shells of more intri- 

 cate pattern than their predecessors. Among the new Gasteropods, 

 that came at that time and have staid to the present time, is the Limpet 

 or Patella family. Two other families, the Haliotidae, embracing the 

 Pleutomaria and Murchisonia, and the Atlantidae family, including the 

 Bellerophon and Cyrtolites, came in at this time also, but became extinct 

 in the Carboniferous era. ' ' A whole family, in the case of the Grap- 

 tolites, approaches its extinction." "Nearly all the genera of the Cys- 

 tideans also became extinct." This singular type of Crinoids had its 

 climax in the Lower Silurian, though not its final extinction ; after this 

 its species were few, while there is a great increase of Crinideans. * 



UPPER SILURIAN. 



Now passing into the upper silurian we have a continuation of the 

 same story. 



A crustacean, which might be taken for a cross between a worm and 

 a cray-fish, made its appearance about the close of the Upper Silurian. It 

 is called Eurypterus remipes. 



" The range of animal life was, in its grander divisions, the same as in 

 the later part of the Lower Silurian." Nevertheless, great changes are 

 constantly going on through the disappearance of many species, genera 

 and even families, and the introduction of others. The details of fam- 

 ily life are entirely remodeled, while the general structures remain. 

 ' ' Not a species existed in the latter half of the Upper Silurian that was 

 alive in the latter half of the Lower Silurian. Less than a dozen species 

 * Dana 225. 



