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. 
Diam. .... 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 Iunule, 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 
H 
