Silurian Age of Molluslcs. 79 



are continued into the Devonian, and these disappear long before the 

 close of that age. " The number of species thus extinguished from the 

 latter half of the Lower Silurian to the Devonian, is estimated at 880. t 

 These are species, not families or orders. This general extermination 

 extends, however, by the close of the Upper Silurian era to many of the 

 genera as well as their species. Moreover, ' ' To the list of existing 

 genera no additions are made in the course of the Upper Silurian. All 

 but the few before enumerated, viz. , || Lingula, \\ Discina, I Nautilus, \\ Rhyn- 

 conella, J Pleurotomaria, and || Crania, become extinct." 



This is American history; but the European is much like it. Many 

 genera are common to both continents. ' ' Similar genera make their 

 appearance and others their exit. In neither have we any evidence 

 that the progress had reached to the introduction of land or fresh water 

 species of animals, and no relic of a land plant has yet been discovered 

 in the Silurian strata of Europe or Britain, except in the uppermost 

 beds." The number of European species was also large in the Upper 

 Silurian, reaching over 2,000 in Bohemia. The Upper Silurian "was an 

 era of small areas of dry land, of continents mostly submerged, though 

 not necessarily at great depths, of warm waters to the poles, of marine 

 life, of Mollusks and inferior Crustaceans as the higher life of the seas, 

 and the flower-like Corals and Crinoids as the inferior life, and of Sea- 

 weeds as the vegetation." (Dana 265.) 



Table of whole number of Silurian species, according to Barrande, up to 1872. 



Sponges, and other Protozoans, 153 



Corals 718 



Ecliinoderms 588 



Worms ' 185 



Trilobites 1,579 



Other Crustaceans 348 



Bryozoans (polyzoa) 478 



Brachiopods 1,567 



Lamellibranchs 1,086 



Heteropods ) oqn 

 Pteropods ] ' 



Gasteropods 1,306 



Cephalopods 1,622 



Fishes 40 



Uncertain 14 



Total 10,074 



All the species of the Silurian age are now extinct, probably half of 

 them became extinct during the Silurian age itself. 

 Classification of Plants. 



I. Cryptogams (have no seeds only spores). 



1. Thallogens wholly cellular, (1) Algae, (2) Lichens. 



2. Anoyens wholly cellular, short stems, (Musci) Mosses, Liverworts. 



3. Acrogens (1) Ferns, (2) Ly copodia or Ground Pine, (3) Equiseta horse-tails, scouring 

 rushes (many coal genera). 



II Phsenogams (distinct flowers and seed). 



1. Gymnosperms flowers very simple, and the seed naked, the seed being ordinarily 

 on the 3 nner surface of the scales of cones. Exogenous rings of growth, Pine, Spruce, 

 Hemlock, &c. Includes (1) Conifers, (2) (Cycads), (3) Sigillarids woody fibre without ducts. 



2. Angiosperms Dicotyledons seed of two lobes. Regular flowers and covered seed, 

 Exogenous bark and rings, Maple, Elm, Apple, Rose, and shrubs and trees generally. 



3. Endogens Monocotyledons seed not divided flowers and seed growing inside, no 

 annual rings no bark, new fibres formed among the old ones Palms, Rattan, Reed, Corn 

 Grasses, Lily. 



tThis estimate is now known to be much too low. $A Gasteropod. A Cephalo- 

 pod. II Brachiopod families. 



