OF THE FAMILY OF NAIADES. 431 



same as cardinal tooth ; posterior cicatrices distinct, the smaller 

 one being placed immediately over the large one, and against 

 the lateral tooth; anterior cicatrices distinct; dorsal cica- 

 trices situated on the under part of the cardinal tooth, very 

 perceptible ; cavity of the beaks small ; nacre flesh-red, vary- 

 ing from this to nearly white ; iridescent in the posterior mar- 

 gin. 



Remarks. — This is variety a of U. ellipsis, descril)ed in a 

 former paper, and appi'oaches it closely. Having seen seve- 

 ral specimens since that description v\'as made, my doubts 

 have been satisfied, and I now consider it a new species. It 

 differs from the ellipsis in having the furrow, in being gene- 

 rally covered with fine hair-like rays, and in being always 

 more or less flesh-red inside. I have two specimens of this 

 species which present a singular formation of the posterior 

 basal margin, which is dentate, the points interlocking and 

 almost hooked. The elevation anterior to the furrow com- 

 mences to swell one-third of the distance from the margin 

 to the beaks, increases as it approaches the margin, and as- 

 sumes this dentation, which being successive as the shell 

 increases displays laminae of these dentations in the epider- 

 mis. In the interior this part of the shell has the appearance 

 of having been gouged out. It is exceedingly curious, being 

 the only specimen of fluviatile shells I have seen with a mar- 

 gin approaching to a dentate appearance. 



'5. Unio Planulatus. Plate IX. fig. 13. 



Testa inaquilaterali, ovato-elliptica, transversa; complanatd per iim- 

 bones h natibus usque ad marginem inferiorem, maculis quadratis radi- 

 atim pictd ; natibus prominulis ; dente cardinali parvo, laterali magno, 

 crasso, curvato; margaritd sub-cceruleo-alba. 



Shell inequilateral, ovate-elliptical, transverse, flattened across the 

 umbones from the beaks to the basal margin, marked with square spots in 

 form of rays; valves tiiick ; beaks slightly prominent; cardinal tooth 

 small ; lateral tooth large, thick and curved ; nacre bluish white. 

 VOL. III. 5 R 



