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which has been withdrawn wholly behind the beaks, he employs the term 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

Fic. 599. its apex at the umbonal point 
Homomya calciformis, Ag. With well-preserved external, 5 c 3 
opisthodetic ligament. Lower Oolite; near Bayeux. 2/3. and Its base at the hinge line, as 
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, Arca, Fossula), either 
amphidetically or upon the posterior limb of the cardinal margin. This is 
designated as multiwincular, 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 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. 
