THE LOWER SILURIAN PERIOD. 



in 



striated edges, sometimes sword-like or scimitar-shaped, but 

 often more or less branched ; and it consists of two layers of 

 cells, separated by a delicate membrane, and opening upon 

 opposite sides. Each of these little chambers or " cells " was 

 originally tenanted by a minute animal, and the whole thus 

 constituted a compound organism or colony. 



The Lamp-shells or Brachiopods are so numerous, and pre- 

 sent such varied types, both in this and the succeeding period 

 of the Upper Silurian, that the name of " Age of Brachiopods " 

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

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

 many different forms of Brachiopods which present themselves 

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

 genera Ortliis, Strophomena, and Leptccna for illustration, as 

 being specially characteristic of this period, though not exclu- 

 sively confined to it. The numerous shells which belong to 

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



Fig. 51. Lower Silurian Brachiopods. a, Strophomena alternata, Cincinnati Group, 

 America; ft, StrophomenctfiUtexta, Trenton and Cincinnati Groups, America ; c, Or- 

 this testudinaria, Caradoc, Europe, and America ; d, rf', Orthis plicatella, Cincinnati 

 Group, America ; e, e f , e", Leptcena gertcea, Llandeilo and Caradoc, Europe and 

 America. (After Meek, Hall, and the Author.) 



and fig. 51, c and d), are usually more or less transversely- 

 oblong or subquadrate, the two valves (as more or less in all 

 the Brachiopods) of unequal sizes, generally more or less con- 

 vex, and marked with radiating ribs or lines. The valves of 

 the shell are united to one another by teeth and sockets, and 

 there is a straight hinge-line. The beaks are also separated 

 by a distinct space ("hinge-area"), formed in part by each 

 valve, which is perforated by a triangular opening, through 



