662 



MOLLUSCOIDEA. 



which, in the ventral valve, is partially covered by a deltidium. The 

 ventral valve may be convex or concave, and the dorsal valve follows 

 the curvature of the ventral valve. The species of the genus Stro- 

 phomena are very abundant in the Ordovician, Silurian, Devonian, 

 and Carboniferous formations, often attaining a large size ; but they 

 do not seem to have survived the close of the last-named period. 



Speaking generally, the species of Strophomena may be distin- 

 guished from those of Orthis and Leptazna — both of which they 



Fig. ^i.—Strophoniena antiquata. Ordovician and Silurian. 



much resemble occasionally — by attention to the following points : 

 The shell is usually flat and semi-oval, its length and breadth being 

 about equal, and the hinge-line always equalling and often exceeding 

 in length the transverse diameter of the shell. Rarely, the shell is 

 bent and transversely extended, as in Leptezna. The cardinal pro- 

 cess is large and bifid ; and the muscular impressions are quadrate 

 and laterally expanded. 



The name of Strophodo?ita has been given to forms of Strophomena 

 in which the hinge-line is crenulated, and there is no fissure in the hinge- 



Fig. 518. — Plectambonites {Strophomena) rkomboidalis. Silurian 



area of the ventral valve. The name of Plectambonites (Leptao-onla), 

 again, has been given to forms of Strophomena in which the shell is con- 

 cavo-convex, transversely semicircular, radially striated, and often con- 

 centrically wrinkled in the neighbourhood of the beaks (fig. 518). The 

 valves in these types are generally strongly geniculated, the umbonal 

 region being flattened, while the margins of the shell are bent towards 

 the dorsal aspect. Forms belonging to this group are found in the 

 Ordovician, Silurian, Devonian, and Carboniferous formations ; a typical 



