THE BRACHIOPODA 27 



surface of the ventral valve becomes bent on itself at 

 about right angles, making that valve in the adult highly 

 convex externally, while the ventral valve is correspond- 

 ingly concave, so that the change does not result in great 

 increase of thickness.* Concavo-convex shells are 

 common among Palaeozoic brachiopods : when, as in 

 Leptana, the dorsal valve is concave, they are said to be 

 normally concavo-convex ; when the ventral is concave they 

 are reversed concavo-convex (or convexi-concave). 



The surface of the shell is ornamented with radiating 

 ridges, crossed by much finer concentric lines. In addition 

 to these there appear at a little distance from the umbo 

 coarser and irregular concentric corrugations, which 

 rapidly increase in size up to the line of growth-change 

 (reflection), beyond which they cease, as though they 

 were premature attempts to make the change in growth- 

 direction. 



On each valve there is a wide and low cardinal area, 

 extending all along the hinge-line. The delthyrium 

 notches both areas equally so that it is rhomboidal in 

 shape. In the ventral valve there is a small deltidium 

 close to the umbo ; in the dorsal a much larger convex 

 plate (chilidium), with a median groove, covers the whole 

 opening. The pedicle-opening is small, between umbo 

 and deltidium (encroaching on the former in old age) ; 

 the pedicle probably rested on the groove of the chilidium. 



In the interior of the ventral valve, well-marked muscle- 



* In Ornithella obovata a somewhat similar growth-change takes 

 place in old age, but that being a biconvex shell, both valves increase 

 in external convexity, and so the shell becomes greatly thickened. 



