MOLLUSCA. LAMELLIBRANCHIATA. 137 



Plicatula. Shell irregular, inequivalve, depressed ; 

 smooth, folded or scaly, without ears, fixed by the umbone 

 of the right valve. Hinge-area obscure. Hinge with 

 strong teeth in each valve ; ligament internal, between 

 the teeth. Adductor impression simple, excentral, near 

 the pallial line. Trias to present day. 



Lima. Shell obliquely oval, equivalve, compressed, 

 with radiating strise or ribs, the ribs scaly. Valves gaping 

 anteriorly and sometimes posteriorly. Umbones distant, 

 sharp, prominent, auriculate, the ears unequal. Hinge 

 straight without teeth. A triangular hinge-area, furnished 

 with a central ligamental cavity. Adductor impression 

 large, divided, excentral. Two small pedal impressions. 

 Carboniferous to present day. 



Pecten. Shell suborbicular or oblong, free, closed, 

 equilateral, inequivalve, umbones approximated. Surface 

 usually with radiating ribs, auriculate, the anterior ears 

 larger than the posterior, the right one with a byssal 

 sinus. Right valve more convex than the left. Hinge-line 

 straight. Ligamental cavity central, narrow, triangular, 

 internal. Adductor impression large, sub-central. Lias 

 to present day. (Possibly also Carboniferous.) 



Aviculopecten. Shell a little inequilateral ; right 

 valve less convex than the left. Anterior ear smaller than 

 the posterior ; a byssal sinus beneath the anterior ear in 

 the right valve. Ligament in a narrow groove parallel to 

 the hinge-line. Adductor impression large, sub-central. 

 Surface generally radiately ribbed. Devonian to Per- 



