5OO TRANSACTIONS OF THE WAGNER FREE 



amphidetic ligament, while Tellina or Venus may serve as examples of groups 

 with an opisthodetic ligament. 



In many bivalves a lozenge-shaped cardinal area extends atnphidetically 

 between the beaks, while the ligament is truly and wholly posterior, being 

 visible as an oblique triangular space with its apex at the umbonal point and 

 its base at the hinge-line, of which arrangement Avicula shows an excellent 

 example. Nearly every stage of the recession of the ligament can be ob- 

 served, from truly central to posterior, in Lima and its allies. In general, in 

 spite of these differences, Neumayr alludes to the species having an amphi- 

 detic area as if the ligament partook of the areal extension. 



No part of the animal has accommodated itself to the dynamic influences 

 attending growth and evolution more effectively than the ligament, and this is 

 probably because strains and stresses are transmitted to it through the rigid 

 valves rather than by the more elastic tissues. The most perfected type of liga- 

 ment is that which may be compared to a cylinder split on one side, attached 

 by the severed edges, one edge to each valve. For this type (Tellina, Psatn- 

 mobia, Cardiuiti) I propose the term parivincnlar ; its long axis corresponds 

 with the axis of motion or vertical plane between the valves. It is always opis- 

 thodetic. Another form is like a more or less flattened cord extending from one 

 umbo to the other (Spondylus, Lima, Avicula), with its long axis transverse to 

 the plane of the valve margins and the axis of motion. This I call alivinc nlar ; 

 it may be central or posterior to the beaks, but, unless very short, is usually as- 

 sociated 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, Fossuld), either amphidetically (Area), or upon the posterior 

 limb of the cardinal margin. This may be designated as ntultivincular. The 

 species of this type begin life with an alivincular ligament. 



In the Arcidce the cardinal area is often covered with a glossy, dark-col- 

 ored coating which might be mistaken for an outspread ligament from which 

 the true ligament, which is set in grooves hollowed out of the area, might be 

 thought to be representative of a resilium. An examination, however, will 

 show that the glossy coating is merely modified epidermis and is not continu- 

 ous between the valves, which are connected only by the multivincular liga- 

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

 may degenerate to its archaic epidermic character, as in some species of Spo n- 

 dylns. 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 hinge (or 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 escutcheon 

 may or may not be homologous with the posterior part of the original area. The 

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



