PLATE IX. 



Orthis tricenaria (page 39). 



Figure 1. Orthis Iricenaria, Conrad. Both valves closed. Dorsal view. 

 " 2. The same specimen ; ventral view. 



" 3. Interior of the ventral valve, shewing the muscular impressions (a), 

 and the ovarian spaces (b). 



" 4. Interior of the dorsal valve, a, the teeth or brachial processes — 

 the small linear cardinal process is seen between them, b, c, 

 the quadruple muscular impressions 



