THE LOWER SILURIAN PERIOD. 



109 



has with justice been applied to the Silurian period as a whole. 

 It would be impossible here to enter into details as to the 



Fig. 48. Ptilodictya falciformis. a, 

 Small specimen of the natural size ; b, 

 Cross-section, showing the shape of the 

 frond ; c, Portion of the surface, enlarged. 

 Trenton Limestone and Cincinnati Group, 

 America. (Original.) 



Fig. 49.-A, Ptilodictya acnta ; B, Ptil- 

 odictya Schafferi. a, Fragment, of the 

 natural size ; b, Portion, enlarged to show 

 the cells. Cincinnati Group of Ohio and 

 Canada. (Original.) 



many different forms of Brachiopods which present themselves 

 in the Lower Silurian deposits ; but we may select the three 

 genera Orthis, Strophomena, and Leptcena for illustration, as 

 being specially characteristic of this period, though not exclu- 



Fig. 50. Lower Silurian Brachiopods. a and a', Orthis h'forata, Llandeilo-Caradoc, 

 Britain and America: b, Orthis Jlabelhiium. Caradoc, Britain : c. Orthis sitbqnadraia, 

 Cincinnati Group, America : c', Interior of the dorsal valve of the same; d, Stropho- 

 mena deltoidea, Llandeilo-Caradoc, Britain and America. (After Meek, Hall, and 

 Salter.) 



sively confined to it. The numerous shells which belong to 

 the extensive and cosmopolitan genus Orthis (fig. 50, a, b, c, 



