438 NEW GENUS AND SOME NEW SPECIES 



tween are supplied with numerous hair-like lines, the whole 

 passing in a curve from the beak to the margin ; cardinal 

 tooth large, irregularly cleft and sulcated ; lateral tooth rather 

 short and thick, in the right valve disposed to be double ; an- 

 terior cicatrices distinct ; posterior cicatrices also distinct, the 

 small one being placed against the termination of the lateral 

 tooth ; dorsal cicatrices situated on the under side of the car- 

 dinal tooth ; cavity of the beaks shallow and rounded ; cavity 

 of the disk small ; nacre pearly white and iridescent. 



Remarks. — Mr Rafinesque first observed this singular and 

 interesting species. He found a single specimen near Evam- 

 ville, Indiana, and described it under the name of U. de- 

 presses which name being preoccupied by Lamarck, I have 

 considered it incumbent on me to give it a new name. Many 

 specimens have come under my inspection, and the shell be- 

 ing a very remarkable one, I am induced, in consequence of 

 Mr Rafinesque's short description and imperfect figure, to 

 give a more full description and a correct figure. It is alto- 

 gether peculiar in its rays and its very compressed beaks ; no 

 species is so fiat over the umbones, and no other species pre- 

 sents, when the posterior slope is held towards the observer, 

 a long ellipsis, the widest part of which is about the centre. 

 In consequence of the beaks being so very much compressed, 

 the junior, when not more than an inch long, is exceed- 

 ingly flat, and the cavity proportionally small. When the 

 shell increases beyond this it seems to become suddenly thick, 

 and its form becomes more rounded towards the margins, 

 consequently the adult is very different in form from the 

 junior, which might easily be mistaken for another species. 

 It is more generally gaping at the anterior margin than the 

 other species. It assimilates closely to the planulatus (de- 

 scribed in this paper), but differs in the rays, the much 

 compressed beaks, and being more hatchet shape. In the 

 last character it resembles somewhat the rubiginosus descri- 

 bed in this paper. It sometimes occurs twice the size of the 

 one represented here. 



