I70 BULLETIN OF THE BUREAU OF FISHERIES. 



Anodontas, which are without teeth, the bilateral symmetry is complete. In some 

 marine bivalves the two shells are essentially different, as in the oyster, where one is 

 concave while the other is flattened and smaller. 



The ligament is composed of two parts ; the dark outer layer is inelastic and con- 

 tinuous with the periostracum of the shell; while the inner part, comprising the bulk 

 of the ligament, is elastic and bears somewhat inappropriately the name of cartilage. 

 The elastic cartilage is confined between the inelastic layer above and the firm hinge of 

 the shell below. It is compressed when the shell is closed. The natural or relaxed 

 condition of the shell is, therefore, open; that is to say, with the valves separated below 

 by about one-half inch. Consequently, the shell is kept closed in life only by an exertion 

 on the part of the animal. This is accomplished by means of two stout bands of muscle 

 fibers, constituting the anterior and posterior adductor muscles, which extend from one 

 valve to the other near each end of the shell. These are firmly attached to the shell 

 at each end, the places of attachment being the conspicuous rounded impressions pre- 

 viously noticed. 



The hinge mechanism is completed by the lunule previously referred to. This is 

 a thin homy covering occupying the space between the valves in front of the beak. 

 Unlike the ligament behind, it is stretched when the shell is open. The lunule doubtless 

 has no especial significance except to serve as a protective covering and to make a firm 

 union of the two valves. 



Besides the two adductor impressions and the pallial line, some smaller muscle im- 

 pressions are apparent. Such are those of the muscles which draw back the foot, or 

 the anterior and posterior retractor muscles. These are small impressions, two in each 

 valve, just above the big adductor impressions and in this mussel {Lampsilis liganicntina) 

 confluent with the latter. The impression of the protractor, or the muscle which aids 

 in protruding the foot, is usually quite distinct and just beneath the anterior adductor 

 impression. Deep in the beak cavity and on the under surface of the cardinal teeth, 

 or the bridge between cardinal and lateral teeth, are small pits which are the points of 

 attachment of numerous small muscles that serve to elevate the foot. These last are 

 the dorsal muscle scars referred to in systematic descriptions. (See PI. XXI, fig. i.) 



DIVERSITY IN FORM. 



Many modifications of the above description would have to be made for other species 

 of mussels. The sheU may be pear-shaped as in the niggerhead {Quadnda ebcnus), or 

 nearly circular as in Quadrula circulus; it may be very much inflated as in Lampsilis 

 capax or in L. ventricosa (the pocketbook), or exceedingly compressed as in Symphynota 

 comprcssa. In some the shell is not only greatly flattened from side to side but also 

 extends upward in wings before and behind the beaks. Such species are given locally 

 such descriptive names as pancakes, hatchet-backs {Lampsilis alata), or heel-splitters 

 (Symphyiwla cowplanata). Some shells are proportionately very heavy, while others, 

 included mostly in the genus Anodonta, the paper-shells or floaters, are so thin as to be 

 useless for any present economic purpose. The Anodontas, adapted to live in lakes or 

 close alongshore in streams, are further characterized by the entire absence of teeth. 



Variations in thickness or in uniformity of thickness are important from the stand- 

 point of the button makers, and so also are variations in the surface sculpture. Some 



