658 



MOLLUSCOIDEA. 



versely 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 

 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 



Fig. 509. — Chonetes Dalmaniana. Carboniferous. 



tubular spines. The species of Chonetes are distributed in the Silurian, 

 Devonian, and Carboniferous periods. 



Family 2. Strophomenid^. — In this great family of Brachio- 

 pods the shell is rounded or subquadrate, generally compressed, 

 plano-convex, concavo-convex, or biconvex, the beaks being rarely 

 prominent, and the hinge-line being straight and long. The shell 

 seems usually to have been attached by a muscular peduncle. There 

 is a low hinge-area in each valve, often with a triangular fissure, 

 sometimes closed by a pseudo-deltidium, beneath the beaks. The 



Fig. 510. — Interior of the dorsal valve (a) and ventral valve (b) of Orthis striahda, from the 

 Middle Devonian of the Eifel. h, Hinge-area; t, Hinge-teeth; a «, Adductor impressions; 

 d, Impressions of the divaricators and peduncle-muscles; o, Ovarian spaces. (After Davidson.) 



ventral valve has two powerful hinge-teeth (fig. 510, /) ; and the 

 dorsal valve has a prominent cardinal process, between two short 

 brachial processes. The interior of the dorsal valve (fig. 510, a) 

 exhibits four adductor -impressions (a), and well-marked vascular 

 impressions which enclose wide ovarian spaces (0). The interior 

 of the ventral valve (fig. 510, b) shows two narrow, centrally-placed 



