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. ^.Ptilodictya falciformls. a, 

 Small specimen of the natural size ; b, 

 Cross-section, showing the shape of the 

 frond ; c, Portion of the surface, enlarged. 



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

 odictya Schafferi. a, Fragment, of the 



Trenton Limestone and Cin 

 America. (Original.) 



natural 



the eel 



ti Group, Canada 



b, Portion, enlarged to show 

 Cincinnati Group of Ohio and 

 (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 Hforata, Llandeilo-Caradoc, 

 Britain and America : b, Orthis Jlabeltnlwn. Caradoc, Britain ; c. Orthis svbqitadrnta, 

 of the dorsal 



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



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, 



