5l8 HISTORICAL PALEONTOLOGY. 



genera Didymograpsus and Dicranograpsus are exclusively, 

 and the genera Diplograpsus, Climacograpsus, and Rastrites are 

 pre-eminently, characteristic of the Lower Silurian period. 

 The genera Graptolites and Retiolites are those which espe- 

 cially characterise the Upper Silurian deposits, though both 

 commence in the inferior division of the series. Corals are 

 exceedingly abundant in many parts of the Silurian series, 

 certain formations, such as the Wenlock Limestone of Eng- 

 land and the Niagara Limestone of North America, being 

 in places so largely composed of these fossils that they have 

 been regarded as ancient coral-reefs. Almost all the Silurian 

 corals belong to the groups of the Rugosa and Tabulate 



The Echinodermata are largely represented in Silurian de- 

 deposits, more especially by Crinoids and Cystideans. The 

 former are extremely abundant, and belong in many instances 

 to peculiar types. The Cystideans are pre-eminently Lower 

 Silurian, though they occur also in the upper division of the 

 series. They are especially characteristic of the Bala or 

 Caradoc period. The true Star-fishes (Asteroidea) are repre- 

 sented even in the Lower Silurian rocks ; whilst the Brittle- 

 stars (Ophiuroidea) are represented by the genus Protaster. 

 The Sea-urchins (Echinoided) are not represented at all except 

 in the Upper Silurian, and there only by the aberrant genus 

 Palachinus. 



The Annelida are represented in the Silurian rocks by the 

 tracks and burrows of Sea-worms (Helminthites and Scolites), 

 and by the tubes of Tubicola (Serpulites, Ortonia, Conchicolites, 

 Cornulites, Spirorbis, &c.) The little spiral tubes of Spirorbis 

 are commonly found in the Upper Silurian rocks attached to 

 the shells of Orthocerata and the like. 



The Arthropoda appear to have been represented wholly by 

 Crustaceans, no Arachnids, Myriapods, or Insects being yet 

 known with certainty to occur. The most important Silurian 

 Crustaceans belong to the Trilobita, Phyllopoda, Eurypterida, 

 and Ostracoda. The Trilobites are extraordinarily abundant, 

 and every subdivision of the Silurian series has its characteristic 

 species. The " primordial Trilobites " are only represented 

 by such forms as Agnostus and O lemts. The Lower Silurians 

 have many types, amongst which Asaphus, Ogygia, Trinucleus 

 Calymene, Cheirurus, Illcznus, and Phacops may be mentioned 

 as the most important, though most of these range into the 

 Upper Silurians as well. The Silurian Phyllopods are toler- 

 ably plentiful in both divisions of the series and belong chiefly 

 to the genera Ceratiocaris, Peltocaris, and Discinocaris. The 

 Eurypterids are represented in the Lower Silurians, but they 



