788 



THE WONDERS OF GEOLOGY. Lect. VIIT. 



Some specimens of Orthoceras attain a large size, being 

 nearly three feet in length, and having seventy septa. 



The Lituites, another modification of this family, bearing 

 a general resemblance in form to the shell of the recent 

 Spirula, are also found in this formation.* 



21. Silurian Annelides and Crustaceans. — The 

 class of articulated animals termed Annelida, consisting of 

 worms with bodies formed of rings or annular segments, 

 and having red blood, of which the Earth-worm and Leech 

 are familiar examples of the naked forms, and the Serpulae 

 of those protected by shells, traces occur in the Silurian 

 deposits : and of the naked, flexible, soft-bodied marine 

 worms, the Nereis and the Gordius, the distinct imprints 

 of several species have been discovered on the surface 

 of the limestones. | 



Trilobites.\ But the most extraordinary feature in the 

 Silurian fauna, is the abundance and variety of a peculiar 

 family of Crustaceans, of which there are no living repre- 

 sentatives, and which is restricted to the palaeozoic for- 

 mations, and almost exclusively to the most ancient fossili- 

 ferous deposits; for while the Silurian rocks teem with 

 the relics of hundreds of species, but few, comparatively, 

 occur in the Devonian and Carboniferous ; while the pre- 

 valent modern forms of Crustaceans, the Lobsters, Crabs, 

 &c. are entirely absent. 



* Some Orthoceratites from the blue marlite of Cincinnati, are par- 

 tially surrounded by an oval bilobed body, twice the diameter of the 

 enclosed shell ; and which is supposed by Mr. I. G. Anthony, who first 

 observed these fossils, to be the soft parts of the animal ; but in the 

 specimens sent to the Geological Society of London, these bodies 

 appear to be extraneous, and to have no organic connexion with the 

 Orthoceratites. Specimens are figured in the Quarterly Geological 

 Journal, vol. iii. p. 256. 



f Medals of Creation, p. 523. 



% Trilobitcs, signifying threc-lobed, from (lie general form of the 

 carapace, or shell. 



