176 BRACHIOPODA 



length towards the ends of the hinge-line. Delthyrium more or less 

 completely closed by a pseudo-deltidium. Muscular impressions 

 faintly marked. Cardinal process divided. Surface usually orna- 

 mented with radial striae. Silurian to Permian. Ex. C. striatella, 

 Upper Ludlow. 



Leptaena. Shell concavo-convex, semi-oval or nearly quad- 

 rangular, ornamented with small radiating ribs, crossed by concentric 

 folds on the flatter parts ; anterior part bent sharply, often at a 

 right angle to the posterior part. Space between the two valves 

 very small. Hinge-line straight, forming the greatest width of the 

 shell. A narrow area on each valve ; the delthyrium of the ventral 

 valve covered by a convex pseudo-deltidium. Umbo of ventral 

 valve perforated by a small foramen except in old individuals. Two 

 strong diverging teeth in the ventral valve supported by lamellae 

 which are continued round the muscular area. Muscular impres- 

 sions : in the ventral valve, two narrow adductors surrounded by two 

 large divaricators ; in the dorsal, two small adductors near the 

 centre of the valve, behind which are two larger adductors. Cardinal 

 process divided. Ordovician to Carboniferous. Ex. L. rhomboidalu, 

 Bala Beds, etc. 



Strophonella. Shell semicircular or semi-elliptical ; ventral 

 valve concave, dorsal valve convex. Hinge-line long, straight. 

 Dorsal area narrower than the ventral ; inner margins of areas 

 crenulate. Muscular area of ventral valve limited by a prominent 

 border. Silurian and Devonian. Ex. S. englypha, Wenlock Lime- 

 stone. 



Strophomena. Shell semicircular or semi-elliptical, orna- 

 mented with fine radiating ribs ; hinge-line straight, forming the 

 greatest width ; dorsal valve convex ; ventral valve convex near the 

 umbo, but concave in the middle. Ventral area conspicuous, with 

 a pseudo-deltidium ; apex perforated except in old age ; dorsal area 

 narrow. Teeth diverging widely, supported by plates, which are 

 produced into ridges nearly surrounding the muscular area ; the 

 latter is divided by a median ridge. Dorsal valve with a ridge 

 separating two large adductor impressions, in front of which are 

 two narrow impressions. Ordovician and Silurian. Ex. S. antiquata, 

 "Wenlock Limestone. 



