LOWER SILURIAN. 



207 



rocks, which also include flags and slates, but the latter in general 

 more sandy, with beds of limestone. In the whole thickness of 6,000 

 feet, there are two beds of limestone, one, of little persistency, the Hir- 

 nant limestone, of 10 feet, and the other the Bala, of 25 feet ; and be- 

 sides, 1,400 feet below the latter, there is a Bala " ash-bed "of 15 feet 

 thickness. Many beds of igneous rocks are intercalated in some 

 regions. In Shropshire, corresponding beds are sandstones, with occa- 

 sional calcareous layers — the Caradoc sandstone of Murchison. 



Near the town of Llandovery in South Wales, there is a series of 

 beds of sandstone and shale, called the Lower Llandovery, which are 

 referred to the Lower Silurian. 



In Bohemia and Bavaria, the Lower Silurian rocks are schists, 

 quartzytes, and conglomerates, the lower part of Stage D of Barrande; 

 in Scandinavia, there are limestones overlaid by slates and flags ; in 

 Russia, in the Baltic provinces, mainly limestones ; in Spain, schists 

 and limestones, with some sandstones. 



The following list of characteristic fossils of the Lower Silurian of Great Britain 

 serves to show the close parallelism in the life of this era between Europe and America. 



The names of species that occur also in North America are printed in small capitals. 



Protozoans. — Sponges, species of Acanthospongia and Clione; Stromatopora striatella 

 D-Orb. 



Radiates. (1) Polyp-corals: Favorites alveolaris Goldf., F. Gothlandica, two 

 species of Heliolites, Halysites catenulata, Petraia subduplicata M'Coy, Syringo- 

 phyUum (Sarcinula) organum Linn. (2) Acahphs : Alveolites (Stenopora) fibrosa, 

 same, variety Lycoperdon, Ptilodictya dichotoma Portl., various Graptolites, of the 

 genera Diplograptus,Pliyllograptus, etc. (3) Echinoderms: Glyptocrinus basalis M'Coy, 

 two species of Pakeaster, id. of Spharonites and JSchinosphcerites, Agelacrinites Buch- 

 ianus Forbes. 



Mollusks. — (1) Bryozoans: Fenestella antiqua, Ptilodictya acuta H., P. dicho- 

 toma, Retepora Hiringeri. — (2) Brachiopods : Lingula Davisii, Orthis testudinaria, 

 O. vespertilio Sow., O. flabellulum Sow., Fig. 388. O. calligramma Dalm., O. 



Figs. 388-394. 



Fig. 388, Orthis flabellulum : 389, 0. elegantula ; 390, Crania divaricata ; 391, Conocardium 

 dipterum ; 392, Asaphus Powisii ; 393, Illaenus Davisii ; 394, Ampyx nudus. 



elegantula, Dalm. (Fig. 389), O. biforata (or lynx, Fig. 326), O. striatula Con., 

 O. Porcata, Strophomena complanata Sow., Lept^ena sericea (Fig. 331), Crania 

 divaricata M'Coy (Fig. 390), Discina {Trematis) punctata Sow. (near T. cancellata Sow., 

 of the Trenton). — (3) Lamellibranchs : Modiolopsis modiolaris, M. expansa Portl., 



