NAIADES. MOLLUSC A. ALASMODON. 113 



Shell transversely oblong, sub-ovate, thin, translucent, very 

 much inflated ; beaks more than one third from the front, ele- 

 vated, inclined forwards and touching at the points ; hinge margin 

 straight, ending in an angle both anteriorly and posteriorly ; an- 

 terior end narrowest, compressed, especially above, rounded, and 

 widely gaping ; posterior end having its point angular in conse- 

 quence of a sharply angular ridge which passes from the beaks 

 and terminates there, and encloses a broad, depressed space, with 

 the margins compressed into a keel ; base regularly rounded. 

 Surface tolerably regular ; epidermis lying in fine loose folds 

 about the posterior end, color olivaceous, rather yellowish in 

 shells of the ordinary size, and finely radiated and zoned with 

 dark-olive over every part of the shell. Interior a very delicate 

 rose-color, or deep salmon-color, tinted with rose-red. Cardinal 

 teeth compressed, striated, directed forwards, and nearly parallel 

 with the hinge margin ; lateral teeth short ; cavity of the beaks 

 capacious. Length 2J inches, height 2 inches, breadth 1 j inch. 



Some specimens are found much larger. I have one, from 

 Pennsylvania, which measures 4; 2J ; If. Such shells become 

 much thickened and proportionally elongated posteriorly ; they lose 

 the radiations of the epidermis, which is of a dark olive-color. I 

 do not know that any such shells have been found in Massa- 

 chusetts. 



This shell is very rare, and I do not know of its having been 

 found anywhere except in the Plymouth ponds. When young, it is 

 scarcely to be distinguished from U. cariosus ; but it is more in- 

 flated, and the radiations of the epidermis are finer, and cover 

 more of the shell ; it is not so glossy, and its interior is more 

 colored. 



GENUS ALASMODON, SAY. 



Shell transverse, inequilateral ; hinge like that of U'nio, ex- 

 cept that it is destitute of a lateral tooth. 



ALA'SMODON ARCUA'TA. 



Shell more or less kidney-shaped, very inequilateral, thick, 

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