Ch. XXVIL] LOWER SILURIAN ROCKS. 439 



as brachiopods which, abounded in Siluria, and had a very wide geo- 

 graphical range, being met with in the same place in the Silurian series 

 of Russia and the United States. Among 

 its fossils, too, Tentaculites annulatus (fig. 

 590), an annelid probably allied to Ser- 

 pula, is exceedingly common. This also 

 is a link to connect it with the Lower 

 rather than the Upper Silurian. All the 

 shelly sandstone of the Malvern and Ab- 

 beriy Hills, of Tortworth in Gloucester- '^SSSS^ffSSSS^' 



shire, and of the centre of the May Hill Eastnor Park ; nat. size and ma#- 



and Woolhope districts belong to this 



Middle Silurian, which in the Malvern range attains a thickness of 600 

 feet. Of the same age are dense masses of sandstone with shale, 2000 

 feet in thickness, in the higher and disturbed regions of North Wales, as 

 in the Berwyn Mountains for example. According to Professor Sedg- 

 wick the hard quartzose Coniston Grits of Westmoreland may also be 

 referred to the same period. 



LOWER SILURIAN ROCKS. 



Llandeilo Flags. — The Lower Silurian strata were originally divided 

 Dy Sir R. Murchison into an upper group, already described, and termed 

 the Caradoc Sandstone, and a lower one, called, from a town in Caer- 

 marthenshire, the Llandeilo Flags. The strata last mentioned consist of 

 dark-colored micaceous flags, frequently calcareous, with a great thickness 

 of shales, generally black, below them. The same beds are also seen a, 

 Builth in Radnorshire, and here they are interstratified with volcanic 

 matter. Above these typical Llandeilo beds, however, the Lower Silurian 

 contains, both in North and South Wales, some strata in which the 

 Pentameri of the Middle Silurian, already alluded to (p. 438), are asso- 

 ciated with species of fossils identical with those in the Llandeilo flags. 

 The corals of the calcareous zone of the Llandeilo belong to the genera 

 Halysites (see fig. 5*79), Helioliles, Petraia, Stenopora, Favosites (fig. 

 580), and others ;* and there are peculiar Crinoids and Cystideans in the 

 same rocks. These last are amongst the most recent additions made by 

 paleontologists to the Radiata. Their structure and relations were first 

 elucidated in an essay published by Von Buch at Berlin in 1 845. They 

 are the Sphmronites of old authors, and are usually met with as spheroidal 

 bodies covered with polygonal plates, with a mouth on the upper side, 

 and a point of attachment for a stem (which is almost always broken off) 

 on the lower (fig. 591, b). They are considered by Professor E. Forbes 

 as intermediate between the crinoids and echinoderms. The Sphaaronite 

 here represented (fig. 591) occurs in the Llandeilo beds in Wales,f as 

 also in Sweden and Russia. 



Examples are not wanting, though very rare, of star-fish in the same 



* Murchison'8 Siluria, p. 178. 



f Quart. Geo]. Journ. vol. vii. p. 11 ; and Mem. Geol. Surv. vol. ii. p. 518. 



