PANAMIC-PACIFIC PELECYPODA 49 



GLOSSARY OF SOME COMMON TERMS USED IN PELECYPOD DESCRIPTIONS 



Adductors. The valves of a pelecypod are closed by the pull of a special set 

 of internal muscles known as adductors; the attachment of these 

 muscles to the inside of the valve leave rounded or elongated 

 impressions or scars (adductor scars), one or two in number; if 

 there is only one scar (monomyarian) as in Ostrea or Pecten, 

 this impression is placed slightly behind the middle; if there are 

 two scars (dimyarian), they are located at the anterior and 

 posterior ends (anterior and posterior adductor scars). 



Amphidectic, With the ligament developed on both sides of the beaks. 



Anisomyarian. With two adductor scars, of which the anterior one is much 

 smaller than the other (Heteromyarian) 



Beak. The tip or oldest part of the shell above the hinge margin; generally 

 pointed and often coiled or curved. 



Byssus. A bunch of horny threads, loose or in compact form, secreted by 

 a special gland of the foot and by means of which some species 

 are anchored to the sea floor or other objects. 



Cardinal area. The surface of the valve between the beak and the hinge 

 margin, often of a triangular shape and partly or wholly covered 

 by the ligament. Ostrea, Area. 



Cardinal teeth. The middle members of the hinge teeth and which lie under 

 the beak and seem to radiate out from there. 



Cartilage. Obsolete name for the resilium; extensively used in French works 

 on conchology. 



Conchiolin. The organic constituent of the shell; left after the lime has been 

 dissolved away by acid. 



Cordate. Shaped like a heart. Often applied to such shells as Cardium, 

 Cardita, Venus. 



Dimyarian. With two adductor muscle scars. 



Dorsal area or slope. The upper surface of the shell between the dorsal 

 margin and the umbonal slope; it is best differentiated if the 

 posterior umbonal slope is carinate or by marked differences in 

 sculpture. 



Edentulous. Hinge line without teeth. 



Equilateral. Valve with both ends alike. 



Equivalve. A shell with both valves alike in shape, convexity and sculpture. 



Escutcheon. A depressed or flattened area (shield-shape) along the pos- 

 terior dorsal side, on one or both valves, and set off from the rest 

 of the shell surface by a ridge or line, or by a change of sculpture. 



External hgament. The visible part of the ligament, usually shows as a 

 rounded, brown mass just behind the beaks. 



Foramen. A round hole or fissure in the lower valve for the passage of the 

 byssal plug. Anomia. 



Heterodont. Hinge with both cardinal and lateral teeth. 



Heteromyarian. Same as Anisomyarian. 



