MOLLUSCA. LAMELLIBRANCHIATA. 145 



Lucina. Shell orbicular or oval, slightly inequilateral, 

 more or less compressed. Lunule distinct. Umbonal 

 area with an oblique posterior furrow. Hinge with 

 usually two cardinal teeth and one or two lateral in each 

 valve, the lateral and sometimes the cardinal are absent. 

 Ligament external, sometimes partly internal. Adductor 

 impressions well-marked, rugose, the anterior elongated 

 within the pallial line, the posterior oblong. Margins of 

 valves smooth or finely crenulated. Silurian to present 

 day. 



Cardium. Shell convex, nearly equilateral, cordate 

 or oval, generally closed. Umbones prominent, incurved, 

 nearly central. Surface with radiating ribs, which are 

 often spiny. Margins of valves crenulated. Two cardinal 

 teeth, and one anterior-lateral and one posterior-lateral in 

 each valve. Ligament external. Adductor impressions 

 shallow. Pallial line more or less sinuated. Trias to 

 present day. (Perhaps also Palaeozoic.) 



Cyrena. Shell cordifonn, oval or trigonal ; umbones 

 often eroded. Hinge with three cardinal teeth and one 

 anterior- and one posterior-lateral in each valve. Liga- 

 ment prominent, external. Pallial line usually entire. 

 Lias to present day. Lives in fresh and brackish water. 



Cyprina. Shell orbicular or oval, convex, with 

 concentric stria?; a thick epidermis. Umbones prominent, 

 incurved. Ligament external, prominent. No lunule. 

 In each valve three cardinal teeth and one posterior- 

 lateral. Adductor impressions oval. Pallial line entire. 

 Margins of valves smooth. Lias to present day. 



w. P. 10 



