SILURIAN AGE. 167 



shrubs and trees. These Exogens are called Angiosperms, because 

 the seeds are in seed-vessels ; and also Dicotyledons, because the seed 

 has two cotyledons or lobes. 



3. Endogens. — Eegular flowers and seed ; growth endogenous, the 

 plants having no bark, and showing in a transverse section of a 

 trunk the ends of fibres, and no rings of growth (fig. 198) ; as the 

 Palms, Rattan, Reed, Grasses, Indian Corn, Lily. The Endogens 

 are Monocotyledons ; that is, the seed is undivided, or consists of but 

 one cotyledon. 



Only the salt-water species of plants — Algse, or Sea-weeds — are 

 known to occur in the Silurian. Among the fossil Algse there are 

 two prominent kinds : — 



1. Fucoids. — Eelated to the tough, leathery sea-weeds of sea-coasts, 

 which are called Fuel, and which grow in great profusion in some 

 seas, attaining a length at times of several rods. 



2. Protophytes, or infusorial species. — Mostly unicellular plants. 

 The Diatoms are microscopic species which have a siliceous shell ; 

 and they grow so abundantly in some seas that they are producing 

 large siliceous accumulations. A few of these siliceous species are 

 figured above, in figs. 199 to 204. The Bacillaria (figs. 203, 204) con- 

 sist of rectangular segments that close up or slide on one another, 

 as the figures illustrate. 



II. PALAEOZOIC TIME. 



I. AGE OF MOLLUSKS, OR SILURIAN AGE. 



The term Silurian was first applied to the rocks of the Silurian 

 age by Murchison, and is derived from the ancient name Silures, 

 the designation of a tribe inhabiting a portion of England and 

 Wales where the rocks abound. The rocks occur on all the 

 continents and over much of their surface, constituting strata of 

 sandstone, shale, conglomerate, and limestone ; and the most 

 of the strata abound in fossils, as shells, corals, and other allied 

 forms. 



The subdivisions of the Silurian are not only widely different on 

 the two continents, America and Europe, but also on different 

 parts of the same continent. In American geological history it has 

 been found most convenient to recognize that subdivision into 

 periods and epochs which is derived from the succession of rocks 



