MANUAL OF NATURAL HISTORY. 547 



occur in Belgium, along the banks of the Rhine, in 

 Westphalia, the North of Germany, the extreme 

 parts of Russia, and in the lake region of North 

 America. 



In this formation the first traces of organic life 

 are met with. The vegetable remains are few, and 

 chiefly of the lower orders, as algse, ferns, and 

 horse-tails. 



Eight hundred and forty-five species of animals 

 have been described from the Silurian strata by 

 M. D'Orbigny, and eleven hundred species of In- 

 vertebrata have been discovered in the Silurian 

 system of Bohemia alone, including two hundred 

 and fifty species of Trilobites. Only a few scales 

 and bones of fishes, belonging to the genera Sclero- 

 dus, Pterygotus, Tlielodus, and Onchus, have been 

 detected. In this formation those singular crus- 

 taceans occur, belonging to an extinct tribe, com- 

 prising about six families, and numerous genera. 

 These are the Trilobites, clad in mail, with large 

 shield-shaped heads, and compound, sessile eyes. 

 Among them we notice the Illcenus, of an oval 

 shape, and with the angles of the head rounded, 

 an allied species (Isotelus gigas, Dekay) is eighteen 

 inches in length; Asaphus, with the cephalo- 

 thorax ending in a point on each side ; Bumastus, 

 or the " Barr-Trilobite," covered with undulating 

 imbricated plates, all belonging to the family of 

 Asaphidce, having the power of rolling up their 

 bodies like woodlice ; these are found chiefly in the 

 Lower Silurian group. Here, likewise, we have the 



