428 NEW GENUS AND SOME NEW SPECIES 



ated against the side of the lamellar tooth, near its termination ; 

 the anterior adductor muscle makes a cicatrix also against the 

 end of the cardinal tooth ; dorsal cicatrices under the cardinal 

 tooth perceptible ; cavity of the beaks deep and rounded ; na- 

 cre always more or less salmon colour; slightly iridescent at 

 posterior margin ; whitish on the margin near the adductor 

 muscles. 



Remarhs.- — This is a very distinct species. In its general 

 form it approaches nearest to the securis, which, however, is 

 always white in the nacre, and peculiarly rayed. It is pecu- 

 liar in its reddish brown epidermis, which colour is caused 

 by the salmon nacre showing through it. The character of 

 the cardinal tooth is very peculiar, having a tendency to be 

 single in both valves, while the lamellar tooth is quite equally 

 disposed to be double. All the specimens which I have seen 

 are salmon colour in the nacre. If this should prove univer- 

 sally so, it is the only species which we know to be con- 

 stantly of that colour. 



3. Unio Heterodon. Plate VIII. fig. 11. 



Testa rhomboido-ovata, inaquilaterali, ventricosa ; valvulis tenuibus ; 

 dentibus cardinalibus compressis, latis ; dentibus lateralibus sub-curva- 

 tis, dente laterali valvulcB dextrce, duplici ; natibus prominentibus ; liga- 

 mento sub-brevi ; margarild alba. 



Shell rhomboidal-ovate, inequilateral, ventricose ; valves thin; cardi- 

 nal teeth compressed, wide; lateral teeth slightly curved, the double 

 tooth in the right valve; beaks prominent; ligament rather short; nacre 

 v.hite. 



Hab. Schuylkill and Derby Creek, Pa. 



My Cabinet. 



Cabinet of Mr Mason. 



Cabinet of Prof. Vanuxem. 



Cabinet of Dr Griffith. 



