SILURIAN PERIOD. 519 



are pre-eminently characteristic of the higher beds of the 

 Upper Silurian. Lastly, the Ostracoda are often extremely 

 abundant, and belong chiefly to the genera Leperditia, Pri- 

 mitia, Beyrichia, and Entomis. 



The sub-kingdom Mollusca is very largely represented in 

 Silurian deposits, and the Brachiopods and Tetrabranchiate 

 Cephalopods in particular enjoyed a vast extension and a de- 

 velopment which has never since been attained. The Brachi- 

 opods are so abundant in all parts of the series that the 

 Silurian period has been spoken of as the " Age of Brachi- 

 opods." The chief families are the Strophomenidce, Rhyn- 

 chonellida, Spiriferidce, and Lingulidcz. The genus Pentamerus 

 is especially characteristic of the Llandovery Rocks, or of what 

 has been termed the " Middle Silurian " by Sir Charles Lyell. 

 The Caradoc period is noticeable for the great number of 

 Orthides, mostly belonging to simple plaited forms. The ex- 

 clusively Silurian genera, however, are very few, but Obolus, 

 Siphonotreta, and Trematis are not known to have survived 

 into the Devonian period. Bivalves, such as Modiolopsis, 

 Ctenodonta, Lyrodesma, Ambonychia, Pterinea, Cardiola, &c., 

 are far from uncommon ; whilst the Gasteropods are largely 

 represented by such forms as Pleurotomaria, Metoptoma, 

 Holopea, Cydonema, and Murchisonia. The Heteropods are 

 represented by Madurea, Bellerophon, Cyrtolites, and Ecculiom- 

 phalus ; and the Pteropods abounded under the generic forms of 

 Theca (Hyolithes}, Conularia, Tentaculites ; and Pterotheca. The 

 Tetrabranchiate Cephalopods are extraordinarily abundant ; 

 but they belong almost exclusively to the sections of the 

 Nautilida and Orthoceratidce ; the Ammonitidce being repre- 

 sented only by the genus Goniatites, which has not as yet been 

 recognised in the Lower Silurian deposits. The family of the 

 Orthoceratidce attains here its maximum of development, over 

 one thousand species having been described by M. Barrande 

 from the Silurian basin of Bohemia alone. Their highest de- 

 velopment, however, is in the upper and not in the lower 

 division of the series. 



The sub-kingdom Vertebrata is not known to be represented 

 in the Lower Silurian period at all ; but remains of various 

 fishes have been detected in the Upper Silurian series. In 

 Britain, the earliest fish-remains have been discovered in the 

 Lower Ludlow Shale, and consist of the cephalic bucklers of 

 Pteraspidean fishes. In the well-known stratum at the summit 

 of the Ludlow Rocks, familiar under the name of the " bone- 

 bed," have been discovered the defensive spines on which the 

 genus Onchus has been founded, and the shagreen-scales 



