MANUAL OF NATURAL HISTORY. 549 



forms they assume in strata of the secondary forma- 

 tion. The Gasteropoda, again, are much less numer- 

 ous than the Bivalves. Euomphalus, an extinct 

 genus, is common in the grauwacke limestones, and 

 there are found also about ten species of Turritella, 

 six of Turbo, five of Buccinum, Delphinula, and 

 Patella, and three of Nerita, Capulus, Trochus, aud 

 Phasianella. The bivalve Mollusca are not much 

 more numerous than the spiral Gasteropods ; the 

 great majority of Silurian species appearing to have 

 been either pelagian, like Bellerophon and Ortho- 

 ceras, or inhabitants of deep water, as the Brachio- 

 podous tribes. The bivalve genera of Avicula and 

 Cypricardia occur in the Upper Ludlow strata, and 

 Lingula in the Aymestry limestone. The Brachio- 

 poda constitute about a fourth part of the Silurian 

 Mollusks. Among them we find the Pentamerus, 

 divided into four chambers ; the Spirifer, with long 

 spiral arms ; the Orthis, with straight, narrow hinge ; 

 the A typa, with a short hinge-line ; besides Tere- 

 bratulce, Producti, Gypida, and Chonetes. The only 

 Echinodermatous animal met with, is an Ophiura, 

 belonging to the family Ophiuridce ; and the cup- 

 like Encrinite, belonging to the Crinoid forms, re- 

 markable for the jointed peduncle that supports 

 their bodies. Among the Polypiferous animals, we 

 here find the branched Porites, with rayed cells, and 

 the polymorphous tribe of Favosites ; besides these, 

 the curious Graptolithus, belonging to the family of 

 the Sea-Pens, was an inhabitant of the Silurian 

 Seas. 



