58 CONTRIBUTIONS 



so deeply impressed immediately before the beak, as to 

 give a curve to the margin resembling a fracture. The 

 impressa resembles very closely the figure of L. mitis 

 (Sowerby) . He says in his description, " no lateral teeth." 

 This species has them most distinctly marked. He de- 

 scribes the margin as crenulate ours is beautifully so. 



L.papyracea. Plate 1. Fig. 31. 



Description. Shell orbicular, lenticular, somewhat in- 

 flated, concentrically and finely striated, slightly folded 

 behind and subemarginate ; substance of the shell thin ; 

 lunule cordate ; beaks rather pointed ; lateral teeth scarce- 

 ly perceptible ; anterior cicatrix short ; lines of growth 

 indistinctly marked ; cavity of the shell rather shallow ; 

 margin crenulate. 



Diam. 3-20ths, Length 7-20ths, Breadth 7-20ths, of an inch. 



Observations. This species has a general resemblance 

 to the impressa, but differs in being rather less inflated and 

 in being without the deep impression before the beaks. 

 The posterior fold of the impressa is greater, and the lateral 

 teeth are much larger. In the present species the striae de- 

 crease in number on the posterior part and there become 

 squamose. 



L. lunata. Plate 1. Fig. 32. 



Description. Shell anteriorly oblique, inequilateral, in- 

 flated, concentrically striated and furrowed ; substance of 

 the shell thick ; lunule broadly cordate ; beaks elevated, 



