SILURIAN PERIOD. 517 



4. Salina period (comprising the Guelph lime- ) NQ British ivalent 

 stone and Onondaga salt group). ) 



5 . The Lower Helderberg period (comprising^ The ^ h . rf 

 the Tentacuhte and Water - lime groups the I the ^ ^ gjj^ 

 Lower Pentamerus limestone, the Delthyns L ies(co ^ prisingtheLud . 

 shaly limestone, and the Upper Pentamerus j CTOl m\ 

 limestone). 



In Bohemia, as shown by M. Barrande, the Silurian series 

 likewise admits of a subdivision into an Inferior and a Supe- 

 rior division. The former comprises the single " e'tage D," 

 and is characterised by a fauna to which M. Barrande has 

 applied the term of " second fauna," the so-called "primordial 

 zone" having yielded the "first fauna" of the same palaeonto- 

 logist. The Upper Silurian series comprises the etages E, F, G, 

 and H, and is characterised by the possession of the " third 

 fauna." 



LIFE OF THE SILURIAN PERIOD. In the lower portion of 

 the Cambrian series, as we have seen, organic remains are ex- 

 ceedingly scanty ; but in the upper portion of the same, fossils 

 are tolerably abundant, and belong in part to types which pass 

 upwards into the overlying Silurian series. The fossils of the 

 Silurian series are almost exclusively marine, the only excep- 

 tions being some remains of land-plants, such as stems of 

 Lepidodendron (Sagenaria) and the sporangia of Lycopodiace- 

 ous plants (Pachytheca), the latter having been discovered in 

 the very highest beds of the system. The only other vegeta- 

 ble remains which have been detected in undoubted Silurian 

 deposits belong to what are loosely termed " fucoids " (Licro- 

 phycus, Arthrophycus, Palceophycus, Buthotrephis, &c.), and 

 these are tolerably abundant in various parts of the series. 



The sub-kingdom of the Protozoa is represented by For- 

 aminiferous shells and by Sponges. The latter are tolerably 

 abundant in both the Lower and Upper Silurian and belong 

 to various genera (Palaospongia, Acanthospongia, Astylospongia, 

 Amphispongia, &c.) Here, also, we meet with the singular 

 genera Receptaculites and Ischadites, which have been variously 

 regarded as gigantic Foraminifers, as Sponges, or as inter- 

 mediate forms between the Foraminifera and the Spongida. 



The sub -kingdom of the Coslenterata is represented in 

 Silurian times by the Graptolites and by numerous Corals. 

 The typical Graptolites commence their existence in the Skid- 

 daw and Quebec Groups (Upper Cambrian ?), and are highly 

 characteristic of the Silurian Rocks. If we except the genus 

 Dictyonema, which is probably referable to the Sertularians, no 

 species of Graptolite is known to have survived the close of 

 the Silurian period. Neglecting their earlier appearance, the 



