NEW FRESH WATER AND LAND SHELLS. 207 



Remarks. — This species is allied to the U. occidens, (nobis,) but is a much 

 smaller shell. The three specimens under my examination are all the same 

 size, and might easily be mistaken for young- individuals of the occidens. It dif- 

 fers in the rays, the undulation of the beaks and the size. The rays are slightly 

 interrupted and are visible through the beautiful and delicate nacre. 



Unio atro-marginatus. Plate 13, Fig. 21. \ 



Testa plicatd, oblongd, compressd; valvulis subtenuibus; natibus prominulis; epidermide tenebrosd; 

 dentibus cardinalibus parvis ; lateralibus longis subrectisque; margaritd purpured et valde iridescente. 



Shell plicate, oblong, compressed; valves rather thin; beaks somewhat prominent; epidermis very dark; 

 cardinal teeth small ; lateral teeth long and nearly straight ; nacre purple and very iridescent. 



Hab. Chattahooche River, Columbus, Geo. Dr. Boykin. 



My Cabinet. 

 Diam. .5, Length 1.2, Breadth 1.9 inches. 



Shell folded over the whole disk, oblong, compressed; substance of the shell 

 rather thin; beaks somewhat prominent; ligament rather short and thin ; epi- 

 dermis very dark, almost black ; posterior slope carinate and covered with re- 

 gular, nearly parallel folds ; cardinal teeth small, double in the left and single in 

 the right valve; lateral teeth long and nearly straight; anterior cicatrices dis- 

 tinct; posterior cicatrices confluent; dorsal cicatrices placed nearly in the centre 

 of the cavity of the shell ; cavity of the shell very shallow ; cavity of the beaks 

 very small ; nacre purple and very iridescent. 



Remarks. — Two specimens only of this distinct and interesting species were 

 received among the shells from Dr. Boykin. In outline and in the folds it re- 

 sembles the Margaritana rugosa, {Alasmodonta rugosa, Barnes.) It differs from 

 it not only generically in the teeth, but in the colour of the epidermis, in the 

 nacre and in having folds over the whole disk. The two specimens before me 

 have a beautiful purple nacre shading off into white towards the cavity of the 

 beaks. The folds causing undulations within the cavity display the fine irides- 

 cence of the nacre. The margin is remarkably broad and black. It is very 

 likely that, when numerous specimens are examined, the nacre may be found 

 to vary, and some prove entirely white and some salmon-coloured as in the Unio 

 complanatus. 



VIII. — 3 c 



