BRACHIOPODA. 117 



concave. Ventral valve very convex, often geniculate, 

 sometimes with a median sinus ; umbone large, recurved, 

 imperforate. Surface ornamented with radiating ribs, 

 crossed by concentric folds, especially in the umbonal 

 region. Hinge-line straight, teeth rudimentary. Area 

 linear or absent. Tubular spines, especially in the region 

 of the umbone and ears. Muscular impressions strongly 

 marked. In the interior of the dorsal valve a median 

 process. No brachial skeleton. Devonian to Permian. 



Chonetes. Shell transverse, semicircular, concavo- 

 convex, or sometimes plano-convex ; area double. Hinge- 

 line straight forming the greatest width of the shell. 

 Teeth strong. Upper margin of area of ventral valve 

 furnished with a row of hollow spines, which increase in 

 length towards the terminations of the hinge-line. Surface 

 ornamented with radial striae. Silurian to Carboniferous. 



Strophomena. Shell concavo-convex, semicircular 

 or nearly quadrangular, ornamented with small radiating 

 ribs, sometimes also concentric folds, rarely smooth. Space 

 between the two valves very small. Hinge-line straight 

 forming the greatest width of the shell. Area double, 

 that of the dorsal valve linear, that of the ventral valve 

 with a median notch covered by a pseudo-deltidium. 

 Umbone of ventral valve perforated by a small circular 

 foramen except in old age. Two strong diverging teeth 

 in the ventral valve. Four muscular impressions. Ordo- 

 vician to Carboniferous. 



Leptaena. Shell concavo-convex, semicircular, orna- 

 mented with radiating ribs. Hinge-line straight forming 

 the greatest width. Area double, the dorsal linear. 

 Ventral valve very convex, with two diverging teeth. 

 Dorsal valve with four strongly marked muscular impres- 



