140 MOLLUSCA. LAMELLIBRANCHIATA. 



ternal, lodged in a groove. Posterior adductor large, 

 sub-central ; anterior apical. Devonian to present day. 



Section 3. Homomyaria. 



The two muscles of equal size. Mantle-lobes free or 

 sometimes united posteriorly at one point. 



Area. Shell thick, generally equivalve, sub-quad- 

 rangular, ventricose, costulate or cancellate. Margins 

 smooth or dentate ; closed or open ventrally. Hinge-line 

 straight. Teeth numerous, equal, transverse. Umbones 

 prominent, remote, separated by a lozenge-shaped area, 

 which has numerous ligamental grooves converging from 

 the hinge-margin to the umbones. Adductor impressions 

 subequal, the anterior rounded, the posterior divided. 

 Pallial line simple. Ordovician to present day. 



Cucullaea. Shell rhombic or trapezoidal, ventricose, 

 nearly or quite equivalve. Valves closed, ornamented 

 with radiating striae. Umbones incurved. Hinge-area 

 large, well defined, with diverging ligamental grooves ; 

 hinge-line straight, with two short central transverse teeth, 

 and two to five lateral teeth nearly parallel to the hinge- 

 margin. Ligament external. Posterior adductor impres- 

 sion on a thin raised plate. Pallial line simple. Carbon- 

 iferous to present day. 



Cardiola. Shell thin, convex, oval, generally inequi- 

 lateral ; umbones prominent, incurved. Surface with 

 radiating and concentric grooves. Hinge-line straight ; 

 ligamental area large, horizontally grooved. Muscular 

 impressions unknown. Silurian and Devonian. 



