AND DESCRIPTIONS OF NEW SPECIES. 77 



tooth ; cavity of the beaks deep and angulated ; nacre pearly white and 

 iridescent. 



Remarks. — This species has heretofore been considered as the Ver- 

 rucosus of Barnes. Although the general form resembles the Verru- 

 cosus, it differs from it in several essential characters. It has not the 

 beautiful recurved, and finely undulated beaks, nor has it the dark cho- 

 colate coloured nacre of the true Verrucosus. It has a large interrupted 

 ray across the centre of the disk, which is absent in the other, and the tu- 

 bercles are more in the form of blisters. In the Verrucosus, the first and 

 second growths are furnished with tubercles ; in the Pustulosus, they are 

 not. Objections may be made to the use of colour of nacre as a character. 

 It maybe safely used, when on examination of many specimens, there is 

 no appearance of fading away by tints into another colour. The U. tor- 

 sus (Rafinesque) presents us always with a rich chocolate nacre. The 

 U. rectus (Lam.) varies from rich purple and salmon through all the 

 tints of these colours to perfect white, and the same may be said of 

 the U. «mea/?/s (Barnes) and U. complanatus (Myacomjjlanala^Solan.). 

 The U. circulus (Nob.) varies from perfect white to dark pink. These 

 variations of colours are frequent in this genus. Of the numerous spe- 

 cimens of Verrucosus, I have never seen one which was not chocolate 

 coloured. In the Pustulosus, I have never seen the nacre other than 

 white. 



Unio Stapes. Plate VII. fis;. 8. 



»• 



Testn triangulari, sub(Bquilaterali, postice valde angulatd, tuberculata ; valvu- 

 lis crassis; dentibus cardinalibus subgrandibus; lateralibus brevibus, a cardinalibus 

 separatis ; et versus baseos marginem vergentibus; margaritd alba. 



Shell triangular, nearly equilateral, very angular behind, tuberculated ; valves 

 thick ; cardinal teeth rather large; lateral teeth short, distinct from the cardinal 

 teeth and pointing to the basal margin ; nacre pearly white. 



Hah. Alabama river, Judge Tait. 

 Vol. IV.— U 



