440 



LOWER SILURIAISr ROCKS. 



[Cn. XXVII. 



beds. Brachiopod shells are in the greatest 

 abuudance, chiefl}^ of the genera Orthis^ 

 Leptoena, and Stropliomena (fig. 591). Of 

 the Orthides, those species with broad simple 

 ribs (fig. 592) are particulai'ly characteristic. 

 Such shells as Atry2')a 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 Rliynchonella and Lingula^ 

 genera of which there are living representa- 

 tives in the present seas, were common in 

 the Silurian ocean. 



EchinosphcBrites haltlcus, Eich- 

 wald, pp. (Of the family Cys- 

 tidecB.) 

 a. Mouth. 

 &. Point of attachment of stem. 



Lower Silurian, S. and N. Wales. 



Fig. 592. 



Orthis tricenaria. 



Hall. 



New York. Canada, 



h nat. size. 



"^ig. 593. 



Fig. 594. 



» 



10 



Orthis 'ceapeHUio, Sow, 



Shropshire ; N. and S. 



Wales. 



I nat. size. 



Stropliomena (Orthis) grandis, Sowerby. 

 % nat. size. 

 Horderly, Shropshire ; also Coniston, 

 Lancashire. 



Among the Cephalopoda are OrtJwceratites, with the siphuncle of 

 large dimensions and placed on one side; Siho Lituites (see fig. 577), 

 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 Trlmicleus (figs. 597 and 598) are especially characteristic 



Fis. 595. 



Fig. 596. 



Asaphus tyr annus, Murch. 

 Llandeilo ; Bishop's Castle, &c. 



Ogygia, BucMi, Burm. {Asaphus 

 Buchii, Brongn.) 



Builth, Eaduorshire ; Llandeilo, Caermarthenshire. 



