l68 BULLETIN OF THE BUREAU OF FISHERIES. 



the Unionidae, with which the present paper is concerned, and the Mutelidae of South 

 America and Africa. The Mutelidae differ from the Unionidae in some particulars of 

 structure, especially in the form of teeth on the shell and in the form of larva, which 

 is a lasidium, instead of a glochidium such as has been described above. 



THE SHELL. 



The shell is composed of two parts very similar in exterior aspect, but generally 

 differing from each other in interior form. Each portion is called a valve, and the 

 two valves are hinged together. 



EXTERNAL FEATURES. 



In form the shell is roughly elliptical, evenly rounded in front, but more or less 

 angular behind. The lower or ventral margin is generally evenly rounded, but may be 

 arched inward just behind the middle, especially in shells of females. The dorsal or hinge 

 margin is rather straight except for the rounded prominence on each valve just in front 

 of the middle of the back; this knob, or arched portion of each valve, is called the 

 umbo. Where the umbones of opposite valves approximate each other they are more 

 or less elevated above the surrounding shell surface to form the beaks. The beaks in 

 many species, though not in the mucket, are beautifully sculptured with coarse or fine 

 ridges in the form of single or double loops. With the river mucket, beak sculpture 

 is entirely wanting, while it can be seen clearly in Symphynota complanata (PI. XX, 2d 

 row, 2d fig). Almost every species, if good specimens are available, show some form of 

 beak sculpture;" commonly, however, in older specimens the beaks are so much eroded 

 that the ridges are hardly, if at all, apparent. 



In some streams scarcely a single example can be found with the beaks preserved; 

 in other waters erosion occurs less commonly and the beak markings can be observed 

 even in some of the large examples. 



In some cases the resting periods of winter have left distinct marks by color or 

 otherwise on the shell, so that rings or zones corresponding to the growth of each year 

 are recognizable. The rings of annual growth are not, however, generally recognizable 

 on shells having a dark-colored exterior surface. It is also observed that such rings 

 may result from other causes than the interruption of growth b}' the severity of winter. 

 (See p. 132.) 



A conspicuous feature of the shell is the prominent ridge, which extends from the 

 beaks backward and downward to the posterior ventral angle of the shell. A somewhat 

 similar ridge characterizes almost every species of mussels. 



The exterior color of the shell is a most variable character. Generally speaking, 

 the body color is a greenish straw, relieved by narrow green rays, very narrow on the 

 beaks and widening out toward the lower margin. These raj'S are a nearly constant 

 character in the mucket, but vary in number, in width, in brightness of color, and in 

 being continuous or interrupted. The periostracum, or horny covering, of shells grow- 

 ing in clear streams is generally much more brightly rayed than that of those in turbid 



^ The beak sculpture of young specimens is a very important diagnostic character or means of distinguishing species which 

 may closely resemble each other in general form. Compare the yellow and the slough sand-shells, Lampsilis ajwdotitoides and 

 Lam psilis fatlaciosa , or the pocketbooks, La-nipsihs ventricosa and LavipsUis (Proptera) capax, which are occasionally distinguished 

 by this feature alone. The beak is, of course, the beginning of the shell — the oldest portion. 



