PANAMIC-PACIFIC PELECYPODA 73 



Subclass FILIBRANCHIATA 

 Order EUTAXODONTIDA 

 Superfamily ARCACEA 

 Family ARCIDAE 



To this great family belong the Ark shells, sometimes conservatively 

 referred to a single genus, Area. For our purpose, the arks are divided into 

 three groups or subfamilies, the Arcinae, Anadarinae, and the Noetinae. 

 The Arcinae comprise the true Arks; they are mostly nestlers living under 

 stones or clustered together in cracks and crevices, the shell anchored by 

 its byssus which passes through a narrow or wide opening between 

 the ventral margins of the valves. The Anadarinae and Noetinae are mostly 

 free without a functional byssus or if a byssus is present, it is usually short, 

 the margins of the valves showing little or no gap and fit together closely. 

 The arcoid hinge is taxodont or composed of many small teeth and sockets 

 arranged in a more or less continuous series along a straight hinge line. 

 The ligament is external, attached to a narrow or high cardinal area, usually 

 of triangular shape. Most species are marine but several are brackish and 

 live habitually buried in the soft mud of mangrove swamps and 

 similar situations or at the mouths of rivers emptying into the sea. Such 

 brackish-water species have a coarse, heavy periostracum as protection 

 against the corrosive action of acid waters. 



The ligament of the Arcidae is always external. In most genera of 

 the family, the ligament covers most of the cardinal area or the space 

 between the hinge line and the lateral margins of the umbone and the beak. 

 This elastic structure is made up of two distinct layers; the upper or top 

 layer, usually brown in color, is chitinous in composition, usually thin and 

 functionally corresponds to the tensilium of other pelecypods; it is under- 

 lain by a second or heavier layer composed of vertical, closely packed fibers 

 generally showing a gray or silky luster and color. This lower layer 

 represents the resilium although it lies entirely above the axial fulcrum of 

 the hinge. The area covered by the ligament is set apart on each side by a 

 deep groove into which the tensilium dips and is attached directly to the 

 wall of the shell. Often the surface of the ligament and of the cardinal area 

 beneath it may be plain and smooth (Cunearca) but more often it is cut 

 into by deep lines; in most of the Arcinae and Anadarinae, these in- 

 cisions are in the form of transverse grooves forming more or less trigonal 

 or tent-shaped markings; in the Noetinae, these markings are vertically 

 arranged. These grooves serve for the attachment of ribbons of tensilium 

 which like rubber bands are stretched across the space above the hinge 

 line and through their combined action aid greatly in the operational ef- 

 ficiency of the whole ligament. The cardinal area may be fully or but 

 partly covered by the ligament. The extent of the ligamental coverage 

 should be noted as it is often an important character helpful in identification 

 of a specimen in hand. 



Subfamily ARCDf AE Lamarck, 1809 or Fleming. 1828 

 Shell generally a nestler, more rarely a rock borer, attached by a byssus 



