440 



LOWER SILURIAN" ROCKS. 



[Oh. XXVII. 



beds. Brachiopod shells are in the greatest 

 abundance, chiefly of the genera Orthis 

 Leptcena, and Strophomena (fig. 591). Of 

 the Orthides, those species with broad simple 

 ribs (fig. 592) are particularly characteristic. 

 Such shells as Atrypa and Spirifer, so fre- 

 quent in the Upper and Middle Silurian, are 

 rare or confined to the superior part of the 

 Lower Silurian, while Chonetes and Produc- 

 tus are wholly absent. It is remarkable, 

 however, that Bhynchonella and Lingula, 

 genera of which there are living representa- 

 tives in the present seas, were common in 

 the Silurian ocean. 



EchinospTuzrites baltieus, Eich- 

 wald, sp. (Of the family Cys- 

 tidem.) 



a. Mouth. 



b. Point of attachment of stem. 



Lower Silurian, S. and N. Wales. 



^ig. 593 



Orthis tricenaria, 



Hall. 



New York. Canada. 



i nat. size. 



Fig. 594. 



Orthis vespertilio, Sow. 



Shropshire : N. and S. 



Wales. 



i nat size. 



Strophomena (Orthis) grandis, Sowerhy. 



§ nat size. 



Horderly, Shropshire ; also Coniston, 



Lancashire 



Among the Cephalopoda are Orthoceratites, with the siphuncle of 

 large dimensions and placed on one side; also Lituites (see fig. 5 77), 

 Bellerophon (see p. 407), and some of the floating tribes of mollusca 

 (Pteropods). The Crustaceans were plentifully represented by the Trilo- 

 bites, which appear to have swarmed in the Silurian seas just as crabs 

 and shrimps do in our own. The genera Asaphus (fig. 595), Ogygia 

 (fig. 596), and Trinucleus (figs. 597 and 598) are especially characteristic 



Fig. 595. 



Fig. 596. 



Asaphus tyr annus, Murch. 

 Llandeilo ; Bishop's Castle, &c. 



Ogygia Buchii, Burm. (Asaphus 

 Buchii, Brongn.) 



Builth, Radnorshire ; Llandeilo, Caermarthenshire. 



