PANAMIC-PACIFIC PELECYPODA 41 



tens) may be cited as examples. From all other classes of the Mollusca, the 

 pelecypods are distinguished by their bivalved shell, formed of two, cal- 

 careous valves placed on the sides and thus enclosing and protecting the 

 soft, vital organs of the animal within. The two valves are joined in a flexing, 

 hingelike fashion along their dorsal margins. They differ from most other 

 mollusks in the absence of a true head and of a mouth provided with biting 

 parts, the pelecypods being wholly plankton and detritus feeders. 



As in all other mollusks, the shell of the Pelecypoda is secreted by 

 a fleshy sheet or mantle which covers the soft parts of the animal within. 

 This mantle arises from the back in the shape of two lobes, one on each 

 side, each lobe being closely applied to the inner surface of the valve and 

 directly attached to it along a line near the ventral margin; this is the 

 pallwl line, usually indicated by a more or less continuous linear impression 

 extending from one adductor scar to the other. Each lobe of the mantle 

 functions in the secretion of the shell through the addition of thin, indi- 

 vidual layers to its inner surface thus increasing its thickness and weight, 

 and by similar secretions around the ventral margin adding to its size; 

 therefore, the oldest and thickest part of the shell is located at the beaks 

 while its growth in size is shown by the many small concentric lines (growth 

 lines) which cover its surface, sometimes uniformly distributed, or in con- 

 centric bands or ribbons set apart by deeper impressed lines or by thickened 

 ribs or varices which indicate halts or interruptions in shell growth (resting 

 marks.) 



The teethlike projections along the dorsal margin, together with the 

 elastic ligament, hold the valves together and form a hingelike structure. 

 The ligament may be considered as a fundamental character of the Pele- 

 cypoda (possessed by no other group of the Mollusca) universally present 

 amongst them, although in some cases in a modified or degraded form. The 

 primary function of the ligament is to open or to spread apart the ventral 

 margins of the valves which takes place on relaxation of the adductor 

 muscles. Structurally the ligament is composed of two bands which differ 

 in composition and in the mechanics of operation. In the Veneridae and 

 Tellinidae, these two bands are united in a single structure forming an 

 inner and outer layer. In this case, the ligament is spoken of as "external" 

 and it is visible from the outside just behind the beaks. Therefore, the 

 position of the ligament generally behind or in the rear of the beaks furn- 

 ishes a means of orientation of the whole shell or of an individual valve. In 

 some other families, such as the Mactridae, Myacidae, and the Corbulidae, 

 the two bands of the ligament are widely separated and attached to dif- 

 ferent parts of the hinge plate. In this case, the upper band of the ligament 

 (here called the tensUium,) is much diminished in size and its importance 

 in the opening of the valve much reduced. The scar of the tensilium in this 

 case is placed high on the hinge plate well above the hinge teeth. The larger 

 part of the ligament is now formed by the inner layer or resiliimi, and its 

 attachment to the hinge plate is placed lower, between the hinge teeth, 

 generally in a special groove or pit called the resilifer or chondrophore. The 



