LOWER SILURIAN. 169 



Vermont, are in some places marked with ripples, and have the 

 tracks of worms as well as their borings. In the Potsdam rocks of 

 northern New York and Canada, and those of Wisconsin, there are 

 similar evidences of littoral deposition. Ripple-marks and worm- 

 borings are common ; and, in some places in Canada, there are tracks 

 of Crustaceans, as well as worms (p. 176). In Wisconsin, also, 

 ripple-marks and mud-cracks occur ; and, on some layers, broken shells 

 and other appearances afford the most positive evidence of sea-beach 

 formation. (Hall.) The beds, though of great thickness, are often 

 diagonally laminated, showing the action of tidal currents over the 

 bottom of a shallow sea. The Tennessee and Pennsylvania sand- 

 stones also are, in many places, penetrated by worm-borings, and 

 covered with ripple-marks. 



Economical products. — The Primordial rocks afford much good stone for building, 

 and for the hearths of furnaces, and, in many localities, sand for glass-making. There 

 are gold-bearing quartz veins in the Ocoee series, in Tennessee. 



n. Life. 



The Primordial rocks have afforded evidence only of marine life. 



1. Plants. Algae or seaweeds, of the kind called Fucoids, are the 

 only forms observed. The slabs of sandstone are sometimes covered 

 throughout with vermiform casts of what appear to be stems of this 

 leathery kind of seaweed. Some of the fossils formerly regarded as 

 indications of plants, are now believed to be worm-tracks or borings. 

 But others show by their branching forms that they are true Fucoids. 



2. Animals. The species observed are all invertebrates ; they per- 

 tain to the four sub-kingdoms, Protozoans, Radiates, Mollusks, 

 and Articulates. 



The Radiates were represented by Crinoids ; the Mollusks, by 

 Brachiopods, 1 Pteropods, Gasteropods, and Cephalopods ; and the Ar- 



1 As Brachiopods are the most abundant fossils of the Silurian, their distinguishing 

 characterises and the more important genera are here mentioned, — taken principally 

 from Davidson (Paleontographical Society publications). 



1. Animal. — As stated on page 126, the living animal, unlike all other Mollusks, has 

 a pair of fringed spiral arms, as shown in Figs. 222, 225; and to this the name Brachio- 

 pod alludes, from the Greek for arm and foot. 



2. Shell. — The characteristics of most importance are as follow: — 



a. The large valve (see Fig. 221 and others) is the ventral. 



b. The form of the internal supports connected with the spiral arms varies much; and 

 often they are wanting. The loop-form is seen in Figs. 218, 219, 220; the spiral, in" 

 Figs. 222, 225; the short process, in Fig. 227; and they are wanting in Figs. 230, 231. 



c. The general form and exterior markings of the shell afford important characters; 

 the nearly equal convexity of the two valves, or a median depression on the ventral 

 valve, with a corresponding elevation on the dorsal, Figs. 221, 223. 



d. The beak of the shell may be very large and full (Figs. 221, 238), or very small 

 and little prominent (Figs. 229, 230); :nay have an aperture or foramen at apex (Figs. 

 150, 223, 224), or not. 



