LEPTON. 79 
invisibly punctured, which condition is rather more pronounced 
,withi than on the outside of the apices. 
At one time I almost thought that the L. mitidum 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- 
verum is still unknown, and until it occurs, a safe determina- 
tion on these points cannot be made. The animal of the 
L. nitidum 1 know well, having in the last summer observed 
two examples for four days; it only differs in some minor 
peculiarities from the L. sguamosum, 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 m L. Clarkie the concentric stnie 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; imstead of which, there is a 
minute moveable ossicle, convex on one side and concave on 
the other, as in the Anatine or Thracie; 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 im 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. converum and 
L. nitidum are distinct. Our Leptons, which I name according 
to rarity, will stand thus: Ist, LZ. converum; 2nd, L. Clarkie ; 
brd, L. nitidum; 4th, L. sguamosum. 
One of the distinguishing marks of the new species has been 
already alluded to,—the oblique rounded outline ; but another 
