cr^Ass I 



PELECYPODA 



435 



'Mm 



Fig. 648. 



Hmnoraya'calciformis Ag. Lower Oolite ; near Bayeux. 

 With well-preserved external, opisthodetic ligament. 2/3. 



For the type of ligament which extends on both sides of the beaks, 

 Neumayr adopts the designation amiihidetic ; and for the more perfected type 

 which has been withdrawn wholly behind the beaks, he employs the term 

 opisthodetic (Fig. 648). Glycimeris offers a conspicuous type of the amphidetic 

 ligament : Tellina and Venus ex- 

 emplify the opisthodetic arrange- 

 ment. In many bivalves a 

 lozenge-shaped cardinal area ex- 

 tends amphidetically between the 

 beaks, while the ligament is wholly 

 posterior, being visible as an 

 oblique triangular space, with its 

 apex at the umbonal point and 

 its base at the hinge line, as in 

 Fteria. Nearly every stage in 

 the recession of the ligament can 

 be observed, from truly central to 

 posterior, in Lima and its allies. 



The most perfected type of ligament is that which may be compared to a 

 cylinder split on one side, and attached by the severed edges, one edge to 

 each valve. This type is known as parivincMlar (Tellina, Cardium) ; its long 

 axis corresponds with the axis of motion or vertical plane between the valves, 

 and in position it is iisually opisthodetic. Another form is like a more or 

 less flattened cord extending from one umbo to the other (Spondi/lus, Lima), 

 with its long axis transverse to the plane of the valve margins and the axis 

 of motion. This is called alivincular ; it may be central or posterior to the 

 beaks, but, unless veiy short, is usually associated with an amphidetic area. 

 Lastly, a third form must be noted which consists of a reduplication of the 

 alivincular type at intervals upon the area (Perna, Area, Fossula), either 

 amphidetically or upon the posterior limb of the cardinal margin. This is 

 designated as multivincular, and is developed out of the alivincular type. 



In some forms with a rigid hinge and internal resilium, the ligament may 

 degenerate into its archaic epidermic character, as in some species of Spondylus. 

 It is impossible to draw a sharp line between these and similar forms in which 

 the ligament is not quite reduced to the state of epidermis, as in some species 

 of Ostrea. The cardinal area above referred to is in part the morphological 

 equivalent of the lunule of teleodont Pelecypods. In general, when the 

 ligament has become opisthodetic, the remnant of the area in front of the 

 beaks forms the lunule and may be called prosodetic. The amphidetic area 

 is an archaic feature which has been lost by the more specialised types of 

 modern bivalves, and its gradual disappearance may be traced in various 

 Prionodont genera. 



The separation of the ligament and resilium has been described as due to 

 mechanical causes. In those cases where the resilium becomes submerged 

 between the valves, the area of attachment of its ends in thin-shelled forms is 

 more or less thickened and assumes a spoon-like form projecting from the 

 hinge-plate, termed the cJiondrophore or resiliifer ; this is often reinforced by a 

 special prop or buttress called the clavicle. It has been suggested by Neumayr 

 that part of the armature of the hinge, in the shape of teeth, is due to deposits 

 made parallel to or induced by the presence of the chondrophore and resilium. 



