184 ON THE NAIADES 



Diatu. -8, Length -1, Breadth 1-9 inches. 



Shell scaleniform, cuneated, oblique, very inequilateral, angular be- 

 hind ; substance of the shell thick before, thinner behind ; beaks ele- 

 vated and rounded ; epidermis very finely wrinkled, shining, yellowish 

 brown with numerous green flexuous capillary rays over the whole 

 disk ; ligament rather short and thick ; cardinal teeth elevated, crenate, 

 deeply cleft in the left valve, and emerging from a pit in the right 

 valve ; lateral teeth long and nearly straight ; anterior cicatrices distinct; 

 posterior cicatrices nearly distinct; dorsal cicatrices situated in the 

 centre of the cavity of the beaks ; cavity of the beaks very shallow : 

 nacre beautifully pearly white and iridescent. 



Remarks. — I owe to the great kindness of professor Troost the ex- 

 amination of his select specimens, which he most obligingly sent to 

 me for that purpose. Among them were two specimens of this rare 

 and beautiful species, unsurpassed by any other in the delicacy and 

 exquisite beauty of its rays. In general form it approaches the scah- 

 nius (Rafinesque), but differs from it in the form of the rays altogether. 

 It differs also in colour and in having the beaks less refuse. In dedi- 

 cating this rare and beautiful species to my friend, professor Troost, I 

 do him but an act of simple justice. His constant efforts in the pro- 

 motion of the physical sciences are known and acknowledged, and his 

 investigation in this branch of conchology will do much to illustrate 

 its history in his adopted state. 



Unio PERDix. Plate XI. fig. 31. 



Testa eUipticd, postice subangulald, subsequUate.rali, injfaid, transversa ; valvu- 

 lis subcrassis ; epidermide luteolu, radiis irreguluriter interruptis ; dentibus car- 

 dinalibus elevatis ; lateralibus prope eorum fines majoribus ; margaritd alba et 

 iridescente. 



Shell elliptical, subangalar behind, nearly equilateral, inflated, transverse ; valves 

 rather thick ; epidermis yellowish with irregularly interrupted rays ; cardinal teeth 

 elevated ; lateral teeth larger near their termination ; nacre pearly white and iri- 

 descent. 



