GRAPTOLITES— TRILOBITES. 69 



had already existed and gradually increased to the mul- 

 titudes 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 geo- 

 graphical 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 

 existing 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 Coslenterata, 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 Lamellibranchiata, but 

 has not hitherto adriiitted of any closer fig- 9- 



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 

 appear, as well as the threefold transverse division. The 

 two composite eyes already indicate a high grade of or- 

 ganization. The power of rolling themselves up, which 

 they have in common with several of the crabs now inhab- 

 iting shallow waters and coasts, and likewise their general 

 habit, allow us to infer that they also were denizens of coasts. 



