50 AXEL A. OLSSON 



Hinge. The section of the dorsal margin, held together by interlocking 

 teeth. 



Hinge plate. An inward widening of the margin below the beak and on which 

 the hinge teeth are placed and sometimes the internal ligament. 



Hinge teeth. Toothlike projections along the hinge line. 



Inequilateral. The two ends of the shell unlike in shape and sculpture. 



Inequivalve. Shell with unlike or unequal valves. 



Internal ligament. Ligament when placed largely or wholly within the hinge 

 plate and not easily seen from the outside. 



Isomyarian. With two subequal adductor scars; same as homomyarian. 



Lateral teeth. Hinge teeth in a lateral position, in front or behind the cardinal 

 teeth or the ligament. 



Ligament. An element of the hinge, whose elastic reaction from compression, 

 in opposition to the pull of the adductor muscle force the valves 

 to open or spread apart. 



Lithodesma. A calcified plate placed internally or under the middle portion 

 of the ligament. Characteristic of some genera of the Anomalodes- 

 macea. 



Lunule. A lenticular or heart-shaped area lying below and in front of the 

 beaks, usually weakly or strongly outhned by an impressed 

 line or indicated as a more deeply impressed or sunken area. 

 The lunule may be equally developed in both valves as in the 

 Veneridae, or in one valve only. 



Mantle. The fleshy sheet which envelops the vital organs of the animal 

 within the shell. It is attached to the shell directly along the 

 pallial line and its secretion forms the shell itself. 



Monomyarian. With a single adductor muscle scar in each valve. 



Myophore. An armlike process or apophysis rising from the interior of the 

 shell, usually from the umbonal cavity and serves for the at- 

 tachment of the foot muscles. Found in Pholadidae. 



Nacreous. Shell with the interior having the luster of pearl resulting from 

 the interference of light rays reflected from the surface of many, 

 thin layers with different indices of refraction. 



Nymph. A ridge or lamella of shell placed above the hinge teeth, platelike 

 or with a rugose surface and to which the external ligament is 

 attached. 



Opisthodetic. Behind or facing backwards; applied mostly to the position 

 of the ligament lying behind the beaks. 



Opisthogyrate. Refers mostly to the backward coil of the beaks so that they 

 point backwards or posteriorly. 



Pallial line. The line of adhesion of the mantle to the shell near and gener- 

 ally parallel or concentric with the ventral margin. 



Pallial sinus. An inflexion of the pallial line at the posterior end, its size 

 usually an indication of the size of the siphons or tubular ex- 

 tensions of the mantle which protrude at the posterior end of 



