

 210 MOLLUSCA. 



convex or concave, and the dorsal valve follows the curvature 

 of the ventral valve. The species of the genus Strophomena 

 are very abundant in the Silurian, Devonian, and Carbonifer- 

 ous formations, often attaining a large size ; but they do not 

 seem to have survived the close of the last-named period. 



In the genus Lepttzna are forms smaller than the Stropho- 

 mence, but resembling them in many respects. The shell is 

 more or less completely semicircular (fig. 158), with a double 

 hinge-area, notched in the centre, the 

 fissure in the ventral valve being partly 

 covered by a deltidium. The valves 

 articulate by teeth and sockets, and the 

 surface is generally striated. The Lep- 

 tcBticB extend from the Silurian Rocks 

 to the summit of the Lias, but are not 

 ^.. known in any younger deposits. 

 ; b FAM. VII. PRODUCTID^E. Animal un- 

 of known. Shell entirely free, or attached 

 to submarine objects by the substance 

 of the beak; valves either regularly articulated, or kept in 

 place by muscular action alone. No calcified supports for 

 the oral processes. The Productida are exclusively Palaeozoic, 

 and are especially characteristic of the Devonian, Carbonifer- 

 ous, and Permian deposits. 



The two most important genera of the Productida are 

 Chonetes and Producta. In the genus Chonetes (fig. 159) the 

 shell is concavo-convex, transversely oblong, with a straight 



Fig. 159. Chonetes Dalmaniana, Carboniferous. 



hinge-line. The hinge-line is as wide as the shell, or the shell 

 is eared. The ventral valve is convex, the dorsal concave, 

 and both have a distinct hinge-area, with a central fissure, 

 closed in the ventral valve by a pseudo -deltidium. The 

 upper edge of the hinge-area of the ventral valve is furnished 

 with a row of delicate, tubular spines. The species of 



