LEPTON. 79 



invisibly punctured, which condition is rather more pronounced 

 within than on the outside of the apices. 



At one time I almost thought that the L. nitidum and 

 L. convexum might march together as a single species, subject 

 to many varieties ; but an increase of my series of both has, 

 at least for the present, made me doubt the propriety of 

 considering the two as identical. The animal of the L. con- 

 vexum is still unknown, and until it occurs, a safe determina- 

 tion on these points cannot be made. The animal of the 

 L. nitidum I know well, having in the last summer observed 

 two examples for four days; it only differs in some minor 

 peculiarities from the L. squamosum, one of which I kept in 

 sea-water thirty-four days, when it was killed whilst still 

 vigorous, in consequence of my departure from the sea-side. 



I may state, that in L. Clarkite the concentric striae of 

 increment are close-set and sharp, and sometimes broken into 

 very short waved streaks ; these in some of the specimens are 

 crossed by gently raised lines of an intenser snowy-white than 

 the general colour, which radiate sparingly from the beaks to 

 the basal margin. 



This delicate species cannot be confounded with any of the 

 minuter bivalves : by its hinge it is essentially a Lepton : the 

 nearest approach to any other species is to the Montacuta 

 bidentata, which differs in form, colour, and fragility, and in 

 having the lateral dentitions almost close together, without 

 the primary teeth between them ; instead of which, there is a 

 minute moveable ossicle, convex on one side and concave on 

 the other, as in the Anatirue or Thracia ; this locks into a 

 sloping pit that has sometimes the appearance of being ridged, 

 which is only due to portions of the ruptured ossicle adhering 

 to it ; but in Lepton the primary teeth are persistent or integral 

 parts of each valve. Fresh examples, new facts, and further 

 investigation have all but convinced me that L. convexum and 

 Li. nitidum are distinct. Our Leptons, which I name according 

 to rarity, will stand thus : 1st, L. convexum ; 2nd, L. Clarkite-, 

 3rd, L. nitidum ; 4th, L. squamosum. 



One of the distinguishing marks of the new species has been 

 already alluded to, the oblique rounded outline ; but another 



