no 



HISTORICAL PALEONTOLOGY. 



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

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



Fig. 51. Lower Silurian Brachioppds. a, Strophomena alternnta, Cincinnati Group, 

 America ; b. Strophomena filitexta, Trenton and Cincinnati Groups, America ; c, Orthis 

 testudinaria, Caradoc, Kurope, and America ; d, d ', Orthis plicatella, Cincinnati 

 Group, America ; e, ^. e", Leptcena sericea, Llandeilo and Caradoc, Europe and Ame- 

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



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 

 which, in the living condition, passed a muscular cord attach- 

 ing the shell to some foreign object. The genus Strophomena 

 (fig. 50, d, and 51, a and />) is very like Orthis in general char- 

 acter ; but the shell is usually much flatter, one or other valve 

 often being concave, the hinge -line is longer, and the aperture 

 for the emission of the stalk of attachment is partially closed 

 by a calcareous plate. In Leptana, again (fig. 51, e), the shell 

 is like Strophomena in many respects, but generally compara- 

 tively longer, often completely semicircular, and having one 

 valve convex and the other valve concave. Amongst other 

 genera of Brachiopods which are largely represented in the 

 Lower Silurian rocks may be mentioned Lingula, Crania, 

 Discina, Trematis, Siphonotreta, Acrotrda, RhyncJionella, and 

 Athyris ; but none of these can claim the importance to which 

 the three previously-mentioned groups are entitled. 



The remaining Lower Silurian groups of Mollusca can be 

 but briefly glanced at here. The Bivalves (Lamdlibranchiata) 

 find numerous representatives, belonging to such genera as 



