TO GEOLOGY. 57 



the two great cicatrices raised and very perceptible, the 

 anterior one long ; lines of growth three or four, and very 

 marked ; cavity of the shell deep and rounded, having an 

 impressed furrow across the centre ; margin crenulate. 

 Diara Length 9-20ths, Breadth 9-20ths, of an inch. 



Observations. A single valve only of this beautiful and 

 interesting species has been obtained by me. It does not 

 closely resemble any species recent or fossil which I have 

 seen. The horn or protruding angle on the anterior mar- 

 gin, formed by the bounding ridge of the lunule, at once 

 distinguishes it. The fold or furrow on the posterior part, 

 passing from the beak to the posterior basal margin, forms 

 here a remarkable emargination. 



L. impressa. Plate 1. Fig. 30. 



Description. Shell lenticular, somewhat inflated, nearly 

 equilateral, concentrically striated, folded and emarginate 

 behind ; substance of the shell rather thin ; lunule very 

 small and deeply impressed ; beaks recurved ; the two 

 great cicatrices impressed ; lines of growth numerous ; 

 cavity of the shell rather deep and rounded, having no visi- 

 ble furrow ; margin crenulate. 



Diam. .2, Length .3, Breadth .3, of an inch. 



Observations. This species has some resemblance to 

 the cornuta, but cannot be easily confounded with it. The 

 horn of the one and the deeply impressed lunule of the 

 other eminently distinguish them. This little lunule is 



