566 



FOSSILS OF THE 



[Ch. XXYIL 



slimy seaiment. Some of our most eminent naturalists still hold to 

 this opinion, others refer them to Bryozoa. 



The brachiopoda of the Llandeilo flags, which are very abundant, 

 are in the main the same as those of the Caradoc Sandstone, but 

 the other mollusca are in great part of different species. 



In Europe generally, ; as, for example, in Sweden and Russia, no 

 shells are so characteristic of this formation as Orthoceratites, usu- 

 ally of great size, and with a wide siphuncle placed on one side 

 instead of being central (see fig. 653). The same form also occurs in 



Fig. 653. 



OrtJioceras duplex, Wahlenberg. Eussia and Sweden. 

 (From Murchison's " Siluria.") 

 a. Lateral siphuncle laid bare by the removal of a portion of the chambered shell. 

 &. Continuation of the same seen in a transverse section of the shell. 



the Bala beds in England. Among other Cephalopods in the Llan- 

 deilo flags are Lituites (see fig. 629) ; in the same beds also are found 

 Bellerophon (see fig. 577, p. 52Q) and some Pteropod shells (Conula- 

 ria, Theca, . &c), also in spots where sand abounded lamellibranchiate 

 bivalves of large size. The Crustaceans were plentifully represented 

 by the Trilobites, which appear to have swarmed in the Silurian seas 

 just as crabs and shrimps do in our own. The genera Asaphus (fig. 

 654), Ogygia (fig. 655), and Trinucleus (figs. 646, 647), form a 

 marked feature of the rich and varied Trilobitic fauna of this age. 



Fig. 655. 



Asaphus tyrannus. Murcb. 

 Llandeilo ; Bishop's Castle, &c. 



)ia BucMi, Burm. 

 Syn. Asaphus Buchii, Brongn. 

 Builth, Eadnorshire ; Llandeilo, Caermartbenshire. 



Beneath the black slates above described of the Llandeilo forma- 

 tion, graptolites are still found in great variety and abundance, and 

 the characteristic genera of shells and trilobites of the Lower Silurian 

 rocks are still traceable downwards, in Shropshire, Cumberland, and 



