460 SPECIAL GEOLOGY. 



These fossils were obtained from a .superficial area, not 

 more extensive than one-sixtieth part of the Adriatic ; 

 proving thereby, that the Silurian fauna was not only as rich 

 but as much influenced by geographical conditions as that of 

 any subsequent epoch. The perfect condition of many of 

 these fossils enabled this naturalist to collect a series of the 

 different forms which species assume in the course of their 

 development; specimens illustrating the metamorphosis of 

 nineteen species of Bohemian trilobites belonging to ten 

 genera, have been obtained. M. Barrande has shown that one 

 species, known as Sao Mrsuta, presents twenty different forms; 

 out of which former palaeontologists have made eighteen 

 species, distributed in ten genera. These important researches 

 have established the interesting fact conjectured by other 

 naturalists, that the Trilobitidce passed through a series of 

 embryonic forms between their egg-life and mature condition, 

 just as the Crustacea of our rivers and seas do at the present 

 time. A stratigraphical arrangement of the Bohemian 

 fossils further proves a series of changes in organic life 

 corresponding in chronological order, to those of equi- 

 valent groups previously established for the classification 

 of the Palaeozoic strata of Europe and North America.* 



EXEECISES 



OF THE SILUEIAN AND CAMBEIAN GROUPS . 



State the various collections of these groups, the different 

 localities of their beds, their general characteristics, geogra- 

 phical distribution, and principal organic remains ; and prac- 

 tise the usual mode of question and reply. 



* Sir C. Lyell's Anniversary Address, 1851. 



