202 



PALEOZOIC TIME. 



form ; and these, although smaller species, were probably of superior 

 rank to the Orthocerata. 



Trilobites (Figs. 360-366), continued to be the most common an<* 



Figs. 360-367. 

 361 



Crustaceans. — Fig. 360, Asaphus gigas (XK); 361, a, Calymene Blumenbachii ; 362, Lichas 

 Trentonensis ; 363, Trinucleus concentricus ; 364, 365, Agnostus lobatus (X4);366, same, 

 natural sise ; 367, a, b, Leperditia fabulites (natural size). 



largest of Articulates. Besides, there were many of the little 

 bivalve Crustaceans or Ostracoids, the shell of one species of which 

 is shown in Fig. 367. 



Characteristic Species. 



1. Trenton Epoch. 



1. Protozoans. — Sponges. — Astylospongia parvula B. from the Trenton, near 

 Ottawa City, Canada. Perhaps related to the Sponges, Stromatocerium rugosum H., 

 Black River limestone; and Eeceptaculites globularis H., R. Oweni H., from the Galena 

 limestone of Wisconsin and Illinois. 



2. Radiates — (a.) Polyps. — Fig. 317, Petraia corniculum H., a coral of the 

 Cyathophyllum family, P. profunda H., Trenton limestone; P. aperta B., Black River 

 limestone. Fig. 318, Columnaria alveolata H., Black River limestone, hut occurring 

 elsewhere in the Trenton, — a section of one of the columnar cells shows the tables or 

 partitions of the interior; Fig. 318 a, top-view, showing the radiate cells; Fig. 319, 

 Chcetetes lycoperdon of the Trenton, a solid coral of a conoidal or hemispherical form, 

 having a fibrous or fine columnar structure, as shown in the sectional view, Fig. 320. 

 Stenopora fibrosa Goldf., is a common species ; it began in the Calciferous, and con- 

 tinued into the Upper Silurian. The chain-coral (genus ffalysites, a species of which 

 is shown in Fig. 370) is occasionally found in the Trenton rocks, as in the Galena 

 limestone, and in Canada. Fig. 372, Tetradium fibrosum Saff., Tennessee, Canada, a 

 fine columnar coral with tubular quadrate cells; T. columnare H., Tenn. ; Aulopora 

 arachnoidea H. 



(b.) Acalephs. — Fig. 321, Graptoliihus amplexicaulis H. of the Trenton, of New York 

 and Tennessee; 321 a, an enlarged view. The genera Chcetetes (Fig. 319) and Steno- 

 pora have been referred to the Acalephs. 



(c ) Echinoderms. — Fig. 322, the Star-fish Pakeaster matutina H., of the Trenton: 



