FRESH-WATER MUSSELS. 169 



ones. Young shells are more brightly rayed than old, the rays generally fading some- 

 what or wholly disappearing with age. In different localities, and even in the same 

 bed, the colors are various, the shells may be nearly uniformly straw-colored or largely 

 green; again, a red or rusty-brown color may predominate. The red color without 

 is commonly associated with a pink nacre within. The shell may be smooth and glossy 

 or roughened by fine lines; a silky appearance may be caused by innumerable fine 

 laminae or folds projecting out from the surface of the periostracum. The silky surface 

 is characteristic of some species, as the hickory-nut, Obovaria ellipsis. 



Looking now at the top or hinge of the shell there is seen just back of the beaks a 

 long, narrow, tough, leathery, elastic band, the ligament, an important part of the hinge 

 mechanism. Just in front of the beak is a small region between the shell valves, which 

 is occupied by a similar horny material. This is called the anterior lunule, but in the 

 mucket it is scarcely developed, being about one-half inch long and very narrow in a 

 specimen of 3 inches total length. A posterior lunule may be found just back of the 

 ligament. The compressed form of the shell is noticeable in this view. Roughly speak- 

 ing, the thickness of a mucket from side to side is about one-third of the length, while 

 the width — or height, more correctly — is about two-thirds of the length. 



INTERNAL FEATURES. 



The interior surface of the shell is smooth, white, and lustrous, and usually somewhat 

 iridescent in the extreme posterior portion. In color it is white or pinkish in the mucket, 

 but in other species it may be salmon or purple. Often the proper color is obscured by 

 yellow, greenish, rusty, or salmon-colored stains, resulting from disease, injury, or in- 

 clusion of mud in the nacre. The body of the shell is mainly calcareous, being composed 

 chiefly of a compound of calcium of somewhat the same chemical composition as marble 

 or limestone, but differing in physical structure from either. An account of the struc- 

 ture of shell is given in another place (p. 129). 



The conspicuous features of the interior aspect of the shell are the general con- 

 cavity of each valve; the deeper beak cavities; the dorsal margin roughened by ridges 

 or protuberances known as the "teeth;" two rounded, impressed, and roughened sur- 

 faces, one near each end, the adductor m-uscle cicatrices; and a curved impressed line 

 parallel to the margin of the shell, extending between the two scars just mentioned. 

 This last is the pallial line and marks the attachment of certain muscles of the mantle. 



The two valves, it is noted, are practically identical except for the teeth, which 

 instead of being equal in the two valves, correspond to each other in such a way that 

 the teeth of one valve fit into the spaces between the teeth of the opposite valve. The 

 two valves are t-hereby interlocked so that they can not slide over each other. Heavier 

 teeth characterize the mussels that are adapted to live in strong currents, while weak 

 teeth or the total lack of them mark the species that must live in quiet waters. The 

 teeth in each valve are of two forms; at the anterior or front end are the stout, rough, 

 and somewhat conical cardinal or pseudocardinal teeth ; while behind these, and more 

 or less separated from them, are long, narrow, bladelike ridges, the lateral teeth. On the 

 right valve there is one lateral tooth which exactly fits into the deep narrow furrow 

 between the two slenderer lateral teeth of the left valve. The two valves are practically 

 exact mirror images of each other except for the teeth ; accordingly, in species such as the 



