156 STKATIGEAPHICAL GEOLOGY 



and the only other families represented are the Diplograptidae and 

 Retiolitidse. Sfromatoporoids become abundant in the Silurian, 

 and are associated with corals as important rock-builders. The 

 principal genera are Stromatopora, Labechia, and Clathrodictyon. 



Actinozoa. Corals are very abundant in the limestones and 

 calcareous beds ; most of the Ordovician genera occur, Favosites, 

 Heliolites, Halysites, and Omphyma are very common, and the 

 following commence their existence in the British area : 

 Acervularia, Alveolites, Cystiphyllum, Arachnophyllum, and Stauria : 

 while the following have only been found in the rocks of this 

 system, Goniophyllum, Rhizophyllum, Thecia, and Palceocyclus. 



Echinoderma. Of this phylum the crinoids are by far the 

 most abundant, more than fifty species having been found in British 

 Silurian strata. The following genera are specially characteristic of 

 the Silurian : Callicrinus, Clonocrinus, Crotalocrinus, Dimerocrinus, 

 Herpetocrinus, Marsipiocrinus, Pisocrinus, and Periechocrinus ; and 

 besides these the genera Cyathocrinus, Ichthyocrinus, Eucalyptocrinus, 

 Hapalocrinus, and Taxocrinus make their first appearance. Cysti- 

 deans continue to occur, and the genera Lepadocrinus, Placocystis, 

 Pseudocrinus, Prunocystis, and Schizocystis are specially characteristic 

 of the Silurian period. 



Echinoids, which are first found in the Ordovician of Russia, 

 make their appearance in the Silurian of Britain, where they 

 are represented by the genera Echinocystis, Palwodiscus, and 

 Palcechinus. 



Crustacea. Trilobites, though not quite so numerous as in 

 the Ordovician, continue to be common, most of the species belong- 

 ing to the genera Calymene, Phacops, Encrinurus, Harpes, Acidaspis, 

 Illcenus, Homalonotus, Lichas, Cheirurus, Proetus, and Deiphon, 

 only the last being exclusively Silurian. 



Arachnida. Besides trilobites, another remarkable and 

 extinct order of Arthropods is abundant the Eurypterida, repre- 

 sented by the genera Eurypterus, Pterygotus, Stylonurus, and 

 Slimonia. The Xiphosura, which are allied to the Eurypterida, also 

 occur, but are much less numerous, the principal genera being 

 Hemiaspis, and Neolimulus. 



Brachiopoda are remarkably abundant. Most of the Ordo- 

 vician genera recur, such as Lingula, Orthis, Leptcena, Camarotcechia, 

 and other Rhynchonellidse, Strophomena, Glassia, and Triplecia ; 

 and the following genera make their first appearance : Atrypa, 

 Ghonetes, Dayia, Gyrtia, Cyrtina, Meristina, ? Meristella, Pentamerus 

 (Conchidium), Stricklandia, Rhynchotreta, Wilsonia, Nucleospira, 

 and Spirifer. 



Lamellibranchia. Members of this class also become more 



