58 CONCHOLOGY. 



in a commencing spiral, hinge dorsal, long, similar, formed of 

 two flat cardinal teeth and one lamellous behind the ligament ; 

 ligament dorsal, exterior, diverging anteriorly towards the sum- 

 mit ; muscular impressions very distant and small. Inhabits the 

 Mediterranean and British seas. Four living species. One fossil. 



Isocardia Moltikana. Isocardia cor. 



I. retusum, I. semisulcata. 



FAMILY XI. 



Arcacea. Four Genera. 



1. Genus Area. 

 Ji7iimal. Body thick, slightly variable in form ; abdomen pro- 

 vided with a pedunculated foot, compressed, fit for adhesion, and 

 cleft throughout its extent ; mantle supplied with a simple row 

 of cirrhi and slightly prolonged posteriorly ; buccal tentacula 

 very small and very thin. 



Shell. Somewhat diversiform, but most usually elongated and 

 more or less oblique at the posterior extremity, often very inequi- 

 lateral ; summits moie or less distant and little flexed to the front ; 

 hinge anomalous, straight, or a little flexed, long and formed by a 

 line of short vertical teeth, decreasing from the extremities to the 

 centre ; ligament exterior, wide, nearly as much before as behind 

 the summit ; two muscular impressions united by a band or 

 palleal impression, not very distinct. Inhabits the American and 

 British seas. Thirty-seven living species. Nine fossil. 



Area semitorta. Area inequivalvis. 



A. tetragona. A. tortuosa. 



A sinuata. A. Noae. 



A. cardissa. A. umbonata. 



A. retusa. A. avellana. 



A. ovata. A. ventricosa. 



A. scapha. A. sulcata. 



A. fusca. A. Helbingii. 



A. Domingensis. A. barbata. 



A. trapezina. A. Magellanica. 



