FRESH-WATER MUSSELS. 1 71 



forms are covered with protuberances or knobs in regular or irregular pattern, thus ac- 

 quiring such common names as warty-backs or pimple-backs; while others have strong 

 ridges running obliquely across the shell, as the three-ridge, Quadrula undulata, the 

 blue-point, Q. plicata, and the washboard, Quadrula heros. One species. Unto spinosus, 

 of Alabama, bears long sharp spines on the shell. Diversity of interior color has pre- 

 viously been alluded to. No satisfactory explanation of the colors of nacre has yet 

 been offered. Certain species are almost always white-nacred, as the pimple-back, 

 maple-leaf, and niggerhead. Others are white or pink, examples of the two colors 

 living side by side. Some species have usually a deep purple or salmon nacre, but 

 white-nacred shells of the same species may predominate in particular streams. 



Variations in external color are conspicuous in any collection of shells even from 

 the same mussel bed. Along with shells of uniform color, light or dark, we find shells 

 of glossy surface and brilliantly rayed; the rays may be continuous or variously inter- 

 rupted, sometimes composed of small zigzag markings forming striking and fantastic 

 patterns. In short, the differences'in form and color of shell are unlimited and could 

 not be described, even within the limits of a systematic monograph. 



THE SOFT BODY. 



For observation of the body the mussel may be carefully opened by severing the 

 adductor muscles close to one valve, preferably the left, and gently freeing the soft 

 mantle from the shell as the knife blade is passed from one end of the shell to the other. 

 Removing or bending back the upper (left) valve, the body of the mussel is seen to be 

 almost completely enveloped in a thin mantle corresponding to the interior of the shell 

 inform and size (PI. XXI, fig. i). 



FORM AND FUNCTIONS OF THE MANTLE. 



The mantle is composed of right and left sheets entirely free from each other except 

 along the back where the two sheets are continuous not only with each other but with 

 the body as well. The mantle is, in fact, a double fold from the back of the mussel 

 draped over the body and lining the shell. A thin wing or dorsal extension of the man- 

 tle covers entirely the surfaces of the cardinal and lateral teeth and underlies the liga- 

 ment. 



The mantle is not of uniform character throughout but shows a broad border thicker 

 than the central portion and somewhat muscular. This border along its inner line is 

 attached to the shell through many fine muscle fibers, the attachment of which forms 

 the pallial line on the shell. The border is muscular and, therefore, contractile; the 

 lower or right mantle, which has not been separated from the shell, will have its edge 

 contracted away somewhat from the margin of the valve; generally there is apparent 

 a thin film of homy material which connects the edge of the mantle with the extreme 

 edge of the shell. It is not infrequently the case that in separating the surface of the 

 mantle from the shell a delicate transparent membrane is distinguishable, some parts 

 of which adhere to the mantle and some parts to the shell. Unless, therefore, a rupture 

 has occurred, the mantle normally is actually continuous at the margin with the outer 

 surface of the shell, and probably organically but delicately connected to the inner surface 

 of the shell over its entire surface. 



