SILURIAN SYSTEMFOSSILS. 



29. The SILU'RIAN SYSTEM (from the Si/ures, or Siluri, the an- 

 cient Britons who inhabited the region where these strata are most 

 distinctly developed) is next above the Cambrian. It is subdi- 

 vided into the upper and lower Silurian strata. In its mineral 

 composition it so closely resembles that of the Cambrian that it is 

 often difficult to distinguish them. These strata are entirely o f 

 marine origin, and many of the beds (as the well-known Dudley 

 limestone) are composed of shells, corals, crinoidea, and those pe- 

 culiar crusta'ceans termed trilobites (fig. 4), held together by a 

 calcareous cement. 



Fig. 4. Trilobites* 



30. The presence of these fossil animals is characteristic of the 

 Silurian and Devonian Systems of strata, because they are rarely 

 met with in other situations. They are found entombed in slate 

 and dark limestone. 



Trilobites, from their extraordinary form and appearance, have, for more 

 than a hundred and fifty years, been objects of great interest to the natu- 

 ralist and of wonder to the general observer, and have long been provin- 

 cially termed Dudley insects or locusts. The most common examples con- 

 sist of a convex, oblong body, divided transversely into three principal 

 parts, and longitudinally into three lobes, by two deep, parallel furrows ; 

 from this last character, by which the family is recognised among natural- 

 ists, the name Trilobite (from the Latin Ires, three, and lobvs, lobe) has been 

 derived. These fossils are the carapaces, or shells, of crustaceans, belonging 

 to an extinct family, which comprises many genera, and numerous species. 



The class of crustaceans consists of two groups, namely : those with eyes 



* Explanation of Fig. 4. 1 . A'saphus Caudatus. 2. A'saphus Buchii. 

 3. Caly'mene Blumenbachii. . 



29. How is the Silurian System characterized ? How does it differ from 

 the Cambrian System ? What is the origin of its name ? What are tri- 

 obites ? 



30. Of what systems of rocks are trilobites characteristic ? 



