GRAPTOLITES— TRILOBITES. 69 



fusion of life met with in the Silurian and Devonian 

 strata presupposes an immeasurably long antecedent 

 period during which life had already existed and gradu- 

 ally increased to the multitudes of the Silurian era. 

 We discover in it but scanty remains of marine plants, 

 and only marine animals ; but these are so heterogeneous 

 and varied in form, that they alone would oblige us to 

 infer the existence of coasts, shallow or deep oceanic 

 regions, and a number of geographical conditions on 

 which we see the variety and extent of animal life to be 

 dependent. Besides numerous forms of 

 corals more nearly allied to still exist- 

 ing families, we find the quite peculiar 

 group of Graptolites (fig. 9), which, 

 although not actual polypes, might be 

 ranged next to the so-called Medusa- 

 polypes, and thus justify the inference 

 that preparation was being made for the 

 appearance of the higher forms of the 

 Coelenterata, the Medusae. 



The Articulata are represented by the 

 Trilobites (fig. 10, Trilobites remipes), 

 a crab-like form which recalls the pre- 

 sent group of the Lameliibranchiata, but 

 has not hitherto admitted of any closer definition, as 

 in none of the many thousand specimens examined, 

 of the forms (about 2000) known in the Silurian and 

 Devonian strata, have the legs been preserved. In these 

 three-lobed crabs, the head, trunk, and tail distinctly ap- 

 pear, as well as the threefold transverse division. The 

 two composite eyes already indicate a high grade of 

 organization. The power of rolling themselves up, 



) 



