II 



i 



212 



PALEOZOIC TIME LOWER SILURIAN. 



sionally been seen in the limestones of the Trenton epoch, as in the Galena 

 limestone, and in Canada. 



(b.) Acalep>li8. — Fig. 281 is the Graptolithus amplexicaulis H. of the Trenton, 

 and 281 a an enlarged view. Agassiz refers also to the Acalephs the genera 

 Cksetetes (fig. 279), Favosites, Columnaria (fig. 278), and the related corals having 

 the»cells divided by horizontal partitions, as explained on p. 162. 



(c.) Echinoderms. — Fig. 282, the Star-fish Palseaster matutina H. of the 

 Trenton ; 283, Tseniaster spinosa B. ; fig. 284, the Crinid Lecanocrinus elegans 

 Billings; fig. 285, the two-armed Cystid Pleurocystis filitextus B., of the 

 Trenton, in Ottawa, Canada. (The figure of this Cystid is reversed.) 



The number of Cystids described by E. Billings from the Lower Silurian of 

 Canada is 21; making in all for this era in North America, thus far known, 22 ; 

 the Crinids of the same era amount to 50 species, and the Star -fishes to 11 ; 13 of 

 the Crinids and 8 of the Star-fishes are Trenton species. 



Figs. 286-300. 







•Brachiopods.— Figs. 286, 287, Orthis Lynx; 288, 0. occidentalis ; 289, 0. testudinaria; 29t?. 

 O. tricenaria; 291, Leptama sericea; 292, Stropbomena (Leptaena) rugosa; 293, Stroph. 

 alternata; 294,295, 296, Rhyncbonella increbescens ; 297, 298, Rhynchonella bisulcata; 

 299, Obolus filosus ; 300, Lingula quadrata. , 



3. Mollusks — (a.) Bryozoans.— Species of Retepora and Ptilodictya (re* 

 lated to figs. 266, 267) are common. 



