PELECYPODS 417 



firmly together and are placed one upon either side of the animal. 



The two valves are always held together tightly along a dorsal 



margin by a "hinge ligament," an exceedingly tough, leathery 



substance, and they are usually secured the more firmly by a 



system of interlocking teeth, which project from the hinge. The 



opening and shutting of the valves is controlled by the adductor 



muscles of the animal, by the 



ligament which binds the valves 



together, and by the cartilage, an 



elastic pad of rubber-like appear- 



ance which is lodged just with- 



in the hinge, and is compressed 



when the valves are closed to- 



gether. Thus, when the valves 



are closed, there is always a strain 



upon the adductor to overcome 



the elastic resistance of the car- 



. , TTTl. i j j The interior characteristics of a pelecypod 



tliage. When a peleCypOd dies shell. Left valve of Venusgnidia: .4, anterior, B 



und the pull of the adductor mus- ",, ?' 

 cles is released, the valves gape J'^ffSK ^ 



Open. lu., lunule; u. umbo ; c, cardinal teeth; a. I., au 



tenor lateral tooth ; p. I., posterior lateral tooth. 



The summit or apex of the 



valve is called the umbo, or beak. It is usually twisted to some 

 extent, and in certain forms develops a suggestive spiral figure. 

 The beaks of the valves are the points where the shell-growth 

 begins, the secretion of calcareous matter being made by the man- 

 tle margin and deposited about the edges of the valves; the 

 growth is therefore constantly away from the umbones. 



The umbones usually point forward. In many species they 

 touch each other over the hinge-margin or approximate very 

 closely; in other species the hinge-margin is very wide and the 

 beaks are comparatively far apart. The hinge-margin, lying adja- 

 cent to the umbones, is known as the dorsal margin of the shell, in 

 contradistinction to the ventral margin, opposite the umbones. 

 The anterior margin is the front edge and the posterior margin the 

 hinder edge of the shells, through which the siphons may project. 



When the umbones are about central in respect to the posterior 



27 



