212 



PALAEOZOIC TIME LOWER SILURIAN. 



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

 limestone, and in Canada. 



(b.) Acalephs. — Fig. 281 is the Grcqitolitlms amplexicaulis H. of the Trenton, 

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

 QJisetetes (fig. 279), Favosites, Golumnaria (fig. 278), and the related corals having 

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



(c.) Ecliiiwderms. — Fig. 282, the Star-fish Palieaster matutina H. of the 

 Trenton ; 2S3, Tseniaster spinosa B. ; fig. 284, the Crinid Lecanocrinus elegans 

 Billings ; fig. 285, the two-armed Cystid Pleuroeystis squamosus B., of the 

 Trenton, in Ottawa, Canada. 



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. 



Brachiopods.— Figs. 286, 287, Orthis Lynx \ 288, 0. occidentals ; 289, 0. testudinaria ; 290, 

 0. tricenaria; 291, Leptaena sericea; 292, Strophomena (Leptama) rugosa; 293, Stroph. 

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

 299, Obolus filosus ; 300, Lingula quadrata. 



3. Mollusks — (a.) Bryozoans. — Species of Itetepora and Ptilodictya (re- 

 lated to figs. 2G6, 267) are common. 



