358 MOLLUSCA SUB-KINGDOM vi 



Neumayr adopts the designation amphidetic ; and* for the more perfected type 

 which has been withdrawn wholly behind the beaks, he employs the term 



opisthodetic (Fig. 599). Glycimeris 

 (HHlXt offers a conspicuous type of the 

 amphidetic ligament ; Tellina and 

 Venus exemplify the opisthodetic 

 arrangement. In many bivalves 

 a lozenge - shaped cardinal area 

 extends amphidetically between 

 the beaks, while the ligament is 

 wholly posterior, being visible as 

 an oblique triangular space, with 

 FlG - 5 "- its apex at the umbonal point 



Homomya calciformis, Ag. With well-preserved external, nn ,l :<. VISIQP nt flip Viino-P II'TIP a 

 opisthodetic ligament. Lower Oolite ; near Bayeux. a/ 3 . ancl lts base at tne nin g6 lme > a S 



in Avicula. 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 parivincular (Tellina, Cardium) its long axis 

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

 in position it is usually opisthodetic. Another form is like a more or less 

 flattened cord extending from one umbo to the other (Spondylus, 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 very 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. 1 In cases where the resilium becomes submerged from 

 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 chondrophore 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 



1 Amer. Journ. Science [3], vol. XXXVIII., 1889, pp. 448-451. 



