THE LOWER SILURIAN PERIOD. ICQ 
has with justice been applied to the Silurian period as a whole. 
It would be impossible here to enter into details as to the 
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Fig. 48.—Ptilodictya falciformis. a, Fig. 49.—A, Ptilodictya acuta ; B, Ptit- 
Small specimen of the natural size; 6, odictya Schafferi. a, Fragment, of the 
Cross-section, showing the shape of the natural size; 4, Portion, enlarged to show 
frond ; c, Portion of the surface, enlarged. the cells. Cincinnati Group of Ohio and 
Trenton Limestone and Cincinnati Group, Canada. (Original.) 
America. (Original.) 
many different forms of Brachiopods which present themselves 
in the Lower Silurian deposits; but we may select the three 
genera Orthis, Strophomena, and Leftena for illustration, as 
being specially characteristic of this period, though not exclu- 
Fig. 50.—Lower Silurian Brachiopods. a@ and a’, Orthis béforata, Llandeilo-Caradoc, 
Britain and America; 6, Orthis flabellulum, Caradoc, Britain; c, Orthis subguadrata, 
Cincinnati Group, America ; c’, Interior of the dorsal valve of the same; d, S¢ropho- 
dia gga Llandeilo-Caradoc, Britain and America. (After Meek, Hall, and 
alter. 
sively confined to it. The numerous shells which belong to 
the extensive and cosmopolitan genus O7rthis (fig. 50, a, 3, 6, 
