LOWER SILURIAN. 



187 



The Protozoans were represented by Rhizopods and Sponges ; the 

 Radiates, by Graptolites and Crinoids, but very sparingly by true 

 Polyp corals ; the Mollusks, by species of all the grand divisions, from 

 the lowest, that of Bryozoans, to the highest, that of Cephalopods ; 

 the Articulates by "Worms and Crustaceans, and, among Crustaceans, 

 by a great number and diversity of Trilobites. 



Graptolites, of which some of the forms are represented in the 

 following figures, were exceedingly numerous ; over fifty species have 



^ 288 %{(/JjJV 



Fb. 253, 28£, 285, Graptolithus Logani ; 286,287, Phyllograptus typus ; 288, the young (f 



a Graptolite. 



been found in the Quebec group. These feathery species appear to 

 have grown in immense numbers over the muddy sea-bottom, and, 

 probably, as observed by Hall, in the still waters at considerable 

 depths (probably some hundreds of feet), for they are especially 

 abundant in the fine-grained shales and slates of the era. Each 

 branchlet or stem, as explained on p. 130, was margined with little 

 flower-like animals, looking like polyps. 



Among the Mollusks, Gasteropods (Univalves) were rather com- 

 mon. Figures 290, 291, 292 are some examples. Lamellibranchs 

 (Bivalves) were of several species. Pteropods were numerous and 

 large, vastly larger than any living species of this group. Cephalopods 

 appeared under the form of the straight Orthoceras (Figs. 293, 294, 

 and 295) — a long, tapering shell, chambered like that of the Nautilus. 

 Fig. 295 is a specimen with the extremity broken. The name is from 

 op(9o?, straight, and Ke'pas, horn, and alludes to the form of the shell. 

 Besides these " straight horns," there were also some curved species, 

 and others that were coiled up like the Nautilus of the present day. 



But the most common of all species of Mollusks, by a hundred 

 fold, were the Brachiopods, the characteristic species of the Paleozoic 

 world. Some shells of Lingulellce are two inches or more in length, 

 and resemble much in shape the largest species of Lingula of the 

 present day, though still different. Shells of species of Orthis are 

 very abundant ; one species is represented in Fig. 289. 



