62 CONCHOLOGY. : 
with diverging summits, strongly flexed anteriorly and outwardly, 
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. 
Arcacra. Four Genera. 
1. Genus Arca. Pl. VIII. 
Animal. 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 cirri 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 more 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. Forty-one living species. Nine fossil. 
Arca semitorta. Arca inequivalvis. 
A. tetragona. A. tortuosa. 
A. sinuata. A. Noe. 
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. 
