PALEOZOIC TIME — LOWER SILURIAN. 



505 



There were also many of the minutely columnar Corals, of the Monticuli- 

 pora family, diifering from Chcetetes, to which genus they were formerly 

 referred in having the columns separable. Prasopora lycoperdon, Fig. 646, 

 is a hemispherical species, having the structure shown in Fig. 646 a ; others 

 are branching and foliaceous forms. The branching Corals which form 

 the crystalline points called ''birdseyes" in the Birdseye limestone are 



644-651. 



Eadiates. — Fig. 644, Streptelasma corniculam ; 645, Columnaria alveolata ; 645 a, surface showing cells ; 646, 

 Prasopora lycoperdon ; 646 a, transverse section of same ; 647, portion of Diplograptus amplexicaulis ; 647 a, 

 same enlarged ; 648, Palaeaster matutinus ; 649, Tseniaster spinosus ; 650, Taxocrinus elegans ; 651, Pleuro- 

 cystites fllitextus. Figs. 644, 645, HaU ; 646, 647, Meek ; 648-651, Billings. 



referred to the genus Tetradium, distinguished by its four-sided cell with four 

 points within it, as in Fig. 707, page 511. These peculiar fossils were first 

 called Fucoids by Conrad, and later named Phytopsis cellulosa by Hall, the 

 generic name referring to the resemblance to plants. 



4. Echinoderms include true Crinoids (Fig. 650), Cystoids (Fig. 651), 

 Asterioids, under which are true Starfishes (Fig. 648), and the Ophiuroids 

 or Serpent-star (Fig. 649). 



5. Molluscoids. — Three species of the Trenton Bryozoans are represented 

 on the next page from a memoir by Ulrich (1893); Fig. 652, of a Stictoporella, 

 represents the retiform frond of natural size, and 653, a portion between two 

 of the spaces much enlarged, showing the cells. Fig. 654 is a jointed branch- 

 ing form from Ottawa, Canada, natural size, and 655 represents three joints 

 much enlarged. 



On page 507 are figures of common Trenton Brachiopods of the genera 

 and species named underneath. The figures are mostly from specimens 



