456 



MOLLTJSCA. 



Fig. 307. Leptcena ser- 

 icea. a, Ventral valve ; b, 

 Dorsal valve ; c, Section 

 of the shell. Silurian. 



versely elongated shell, the valves being strongly bent, so 

 that one is always very concave, and the other correspond- 

 ingly convex. Moreover, the muscular 

 impressions are elongated, and the lateral 

 hinge-teeth are confounded in the dorsal or 

 concave valve with the " cardinal process." 

 Finally, the genus Davidsonia includes 

 certain singular Brachiopods, ranging from 

 the Devonian to the Trias, in which the 

 shell is thick and solid, and attached to 

 foreign -bodies by the substance of the 

 ventral valve. The ventral valve has a 

 wide area, with a triangular fissure covered by a convex 

 deltidium ; and though there are no calcified brachial sup- 

 ports, the position of the arms is indicated by two spirally- 

 grooved elevations in the interior of the valve. 



FAM. VII. PRODUCTION. Animal unknown. Shell en- 

 tirely free, or attached to submarine objects by the substance 

 of the beak or by means of spines ; valves either regularly 

 articulated, or kept in place by muscular action alone. No 

 calcified supports for the oral processes. The Produdidce 

 are exclusively Palaeozoic, and are especially characteristic of 

 the Devonian, Carboniferous, and Permian deposits. 



The two most important genera of the Productidce are 

 Chonetes and Product a. In the genus Chonetes (fig. 308) the 



Fig. 308. Chonetes Dalmaniana. Carboniferous. 



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

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

 shell is eared. The ventral valve is convex, the dorsal con- 

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



