NATURAL SCEENCES OF PHILADELPHIA. 471 



panded, its outline is semi-orbicular, flattened somewhat on the upper mar- 

 gin, but quite convex laterally, and moderately so basally, where its junction 

 with the body is not angulated or but slightly so. 



The dorsal edge of the anterior area descends concavely to an acute point, 

 whence the basal edge, sweeping in a quarter circle and thence continuing 

 horizontally, is brought to join the body or fang almost at right angles. The 

 anterior side of the body from this junction is almost straight to the base, its 

 direction being slightly inclined to the posterior side of the valve. The pos- 

 terior lateral edge of the body from its junction with the auricle is continued 

 towards the base, first slightly, but at length becoming decidedly convex in 

 outline, until its somewhat angular junction by a rounded basal margin, with 

 the anterior side. The surface of the body towards the beaks becomes convex 

 and elevated, sloping off towards each side and also towards its dorsal mar- 

 gin, which is mostly higher than either auricle, and convex in outline. The 

 dorsal edge of the posterior auricle is generally somewhat concave in outline, 

 descending slightly from the beaks in typical specimens, although occasionally 

 it is parallel with or even rising slightly above them ; its posterior lateral ter- 

 mination is marked by a slight reflexion upwards, from which the marginal 

 outline of the lateral and basal sides, as before stated, is convex to its junc- 

 tion with the body. 



Viewed internally, the whole dorsal margin of the valve is marked by a 

 raised or thickened border ; the beaks are rather large and overhanging, cul- 

 minating in an irregular tubercle in the centre, from beneath which springs a 

 rather broad curved blade, which terminates in a rapidly enlarging, rounded 

 or irregular clavate end. The inner surface of the fang or body is also 

 marked by an elevated rib, which, not particularly prominent at first, be- 

 comes more distinct as it approaches the base, and is there arrested and 

 turned upon itself apparently by the infolding of the exterior surface, form- 

 ing a rounded tubercle. The division of the posterior auricle from the fang is 

 internally defined by an oblique curved carina, the lower edge of which, near 

 the beaks, slightly projects over the inner disc, but it does not, as in some of 

 the other species, form a continuous ledge from the beaks to the margin. 

 The internal surface of the fang is hollowed in the centre, rising towards either 

 auricle, which becomes convex in the middle and laterally reflected outwards. 

 The surface is pure white and polished. 



Tbe external markings of the valve are very beautiful, the anterior area is 

 ornamented by about sixty close and sharp concentric stria? diverging from the 

 dorsal margin. A narrow radiating area enlarging from the beaks towards 

 the base, occupies the anterior portion of the body and is closely covered with 

 a series of beautiful minute grooves, which define the boundary-line of the an- 

 terior area by their junction almost at right angles with its stria?, these fine 

 grooves, when viewed with a microscope, are found to be decussated by still 

 finer lines. Posteriorly, to this area, the grooves diverge into rather distant 

 slight concentric arches gradually vanishing towards the posterior auricle, the 

 commencement of which is defined by a line, occasionally obsolete. The auri- 

 cle is generally smooth, but occasionally with confluent raised granules or 

 points. The whole surface is white and polished when devoid, as it usually is, 

 of its thin olivaceous epidermis. 



The pallets are somewhat spoon-shaped in outline with a truncate apex. One 

 side is convex and plain, whilst the other is concave, with a raised mid-rib, 

 which, becoming more prominent towards the base, merges into the stalk, 

 which is slender, cylindrical, or flexuous, and about as long as the blade. 



The tube is not much contorted, but generally slightly flexuous, narrow, 

 tapering, polished externally, solid in texture and rather easily detached from 

 its burrow. It is semi-concamerated at its lower end, divided by ten or twelve 

 crowded, thin, orbicular partitions, which, however, leave a large oval orifice 

 in the centre. 



1862.] 



