220 



PALEOZOIC TIME LOWER SILURIAN. 



grown thickly over the muddy sea-bottom ; for the thinnest layers 

 of the slates are sometimes crowdedly covered with their delicate 

 tracery. In the limestone regions of the period, as about Cincin- 

 nati, corals and Trilobites are common, and one species of the 

 latter, related to the Isotelus of Trenton Falls, — the Asaphus (Iso- 

 telus) megistos, — was twenty inches long and a foot broad. 



Characteristic Species. 



1. Radiates. — (a.) Polyps. — No corals have been described from the Utica 

 shale. In the Hudson River beds in New York there are species of Chsetetes 

 related to those of the Trenton, and rarely specimens of the Favistella stellata H. 

 (fig. 335), a columniform coral related to the Favosites, having stellate cells. 

 This species is more abundant in the West. But few of the corals of the Hud- 

 son River epoch from Ohio and the States beyond have been described. 

 Cyathophylla of the genus Petraia occur, as in the Trenton ; also the earliest 

 of the Chain-coral, or Halysites (H. gracilis H., fig. 336, from Green Bay, 

 Wisconsin) ; also Syrinyopora obsoleta H. (fig. 337) ; and species of the genus 

 Tetradium, as Tetradium fibrosum, fig. 338, a. 



Figs. 335-339. 



Fig. 335, Favistella stellata: 336, Halysites gracilis; 337. Syringopora obsoleta; 338, a, 

 Tetradium fibrosum ; 339, Glyptocrinus decadactylus. 



(b.) Acalephs. — Fig. 251 (page 191) represents the Graptolithus pristis H., a 

 species occurring abundantly in the Hudson River and Utica shales at many 

 localities. Several other species have been described by Hall. 



(c.) Echinoderms. — Crinids, Cystids, and Star-fishes occur in the rocks of the 

 period. Among Crinids, the Glyptocrinus decadactylus H. (fig. 339) is not un- 

 common, occurring in New York, Ohio, Kentucky, and other States. Fig. 

 349 represents a large Star-fish from the Blue limestone of Cincinnati, as figured 

 by J. G. Anthony, the original of which was four inches across. 



2. Mollusks. — The Trenton Brachiopods Leptsena sericea, fig. 291 ; Stro- 

 ])homena alternata, fig. 293; Orthis testudinaria, fig. 289; Ortliis Lynx, fig. 286; 

 Orthis occidentalis, fig. 288; Rhynckonella increbescens, figs. 294-296 ; and some 

 others, are continued in the Hudson River period; also the Gasteropods Belle- 



