BRACHIOPODA. 



659 



adductor-impressions (a), flanked by fan-shaped scars (d) produced 

 by the conjoined divaricators and peduncle-muscles, the whole 

 enclosed in a saucer-shaped depression. The arms are devoid of 

 spiral supports, and the shell is usually penetrated by microscopic 

 tubuli, which are comparatively large and remote. 



The three principal genera of the Strop/10 menidce are Strophomena, 



Fig. 5ti. — Orthis callig-ram- 

 ma, yar. Davidsoni ; dorsal 

 and side view. Ordovician. 



512. — Orthis porcata; dorsal and 

 side view. Ordovician. 



Orthis, and Leptcena, and with the doubtful exception of some 

 Mesozoic forms of the last of these, the entire family is confined to 

 the Palaeozoic period. 



In the genus Orthis the valves are articulated by teeth and 

 sockets, and usually are more or less transversely oblong (figs. 



Fig. 513.— a and a , Orthis (Platystrophia) Hforata, Ordovician ; b, Orthis Jlabellulum, Ordo- 

 vician ; c and d, Exterior and interior of the dorsal valve of Orthis subquadrata, Ordovician ; 

 d, Strophomena deltoidea, Ordovician. (After Meek, Hall, and Salter.) 



510-514). There is a straight hinge-line, generally shorter than 

 the width of the shell. Each valve has a hinge-area, notched in 

 its centre by a triangular fissure through which the fibres of the 

 peduncle were transmitted. The shell is often more or less flattened 

 or depressed, and the surface may be smooth, but is more commonly 



