TRENTON PERIOD. 



211 



(b.) Ostracoids, or Bivalve Crustaceans. — Fig. 276, Leperditia Canadensis (var. 

 nana) Jones. 



2. Trenton Epoch. 



1. Protozoans. — Sponges. — Astylospongia parvula B. occurs near Ottawa 

 City, Canada. Several other species of sponge have been described by Rcemer 

 from the rocks of western Tennessee. 



2. Radiates — (a.) Polyps.— True Polyp-corals (pages 162, 163) occur here. 

 Fig. 277 is the Petraia Corniculum, a coral of the Cyathophyllum family, from 



Figs. 277-285. 



Radiates.— Fig. 277, Petraia Corniculum ; 278, a, Columnaria alveolata ; 279, 280, Chtetetes 

 Lycoperdon ; 281, a, Graptolithus amplexicaulis ; 282, Palaeaster matutina ; 283, Taeniaster 

 spinosa: 284, Lecanocrinus elegaus: 285. Pleurocystis squamosus. 



the Trenton limestone. When alive it had probably a circle of tentacles and 

 a flower-like summit, resembling closely fig. 147, and it may have been richly 

 colored. Fig. 278, a fragment of the Columnaria alveolata H., characteristic of 

 the Black River limestone in New York, but occurring elsewhere in the Trenton 

 limestone, — a section of one of the columnar cells shows the tables or par- 

 titions of the interior ; fig. 278 a, top-view, showing the radiate cells ; fig. 279, 

 Ch&tetes Lycoperdon of the Trenton, a solid coral of a conoidal form, having 

 a fibrous or fine columnar structure, as shown in the sectional view, fig. 280. 

 A branching fossil, characteristic of the Birdseye, and called Phytopsis tubu- 

 losus H., is a coral of the genus Tetradium, having cells like fig. 338 a. The 

 chain-coral (genus Halysites, a species of which is shown in fig. 336) has occa- 



