132 Bibliographical Notices. 



atlas, (containing the Trilobites,) is only the first portion of the work, 

 which will be followed by two more (containing the Cephalopods, &c.), 

 forming a complete history of the Silurian system of Bohemia. The 

 text, consisting of more than 900 pages, is devoted (with the excep- 

 tion of the historical introduction and geological sketch) to details 

 connected with the determination of the family of Trilobites, and is 

 illustrated by nearly 2000 carefully engraved figures of all the forms 

 and varieties assumed by the different species. 



The Historical introduction gives a chronological hst of all the 

 authors who have contributed to the knowledge of the palaeozoic 

 formation of the centre of Bohemia, with an analysis of their respect- 

 ive merits and the points of special interest furnished by each writer. 



The Geological sketch, which is merely preliminary to a more 

 detailed one to be given in the third volume, is inserted for the pur- 

 pose of enabling the reader to understand the position of the stratum 

 indicated for each species. 



The Silurian rocks of Bohemia are divided by M. Barrande into 

 an upper and lower system, of four stages each, reposing on one 

 another, and having a conformable stratification, the lower group, 

 A-D, being chiefly siliceous and argillaceous, the upper, E-H, 

 mostly calcareous, and reposing on a trappean base and schists with 

 Graptolites. 



Upper Division. 



H. Argillaceous schists, containing but few fossils. 



G. Argillaceous limestone, with 10 genera and 40 species of Tri- 

 lobites, 10 Brachiopoda, 6 genera and 10 species of Cephalopods. 



F. Mostly calcareous; 10 genera and 75 species of Trilobites, 

 8 genera and 109 species of Brachiopoda, 6 genera and 26 species of 

 Cephalopods. 



E. Trappean and schistose rocks ; 17 genera and 78 species of Tri- 

 lobites ; 22 genera and 220 species of Cephalopods ; 65 species of 

 Brachiopods. 



Lower Division. 



D. Quartzites and schistose rocks; 23 genera and 61 species of 

 Trilobites ; also Cephalopods, Brachiopods, &c. 



C. Protozoic schists, argillaceous schists, with 7 genera and 25 

 species of Trilobites ; other forms very rare. 



A, B. Azoic series, chiefly crystalline and argillaceous schists and 

 conglomerates. 



The divisions C to H are distinguished from one another, (1) by 

 a marked predominance of families or different classes in each of the 

 superposed stages ; (2) in having a very small number of species 

 common to two or more stages. 



The palseontological contrast consists principally in the occurrence 

 of certain genera exclusively characteristic of the lower division C, D, 

 as Paradoxides, Conocephalus, Ellipsocephalus, Sao, Agnostus, Asa- 

 phus, &c. ; also in the development of certain other types, a,slllceniuf 

 and Ampyx, which scarcely pass the limits of this division. ' 



The upper division, E-H, presents other genera which in Bohemia 



