however, the
basal (Pl. I, fig. 6), and occludent margins are slightly reflexed and
prominent. A ridge, generally runs from the umbo to the upper point.
Internally, there is no conspicuous pit for the adductor muscle; under
the umbones, there is generally either on both valves, or only on the
right-hand side (Pl. I, fig. 1 _c_), a small calcareous projection or
tooth, of variable size and shape, even in the same species; it is
generally largest on the right-hand valve; these teeth at first sight
appear to form a hinge, uniting the opposite scuta at their umbones, but
this is not really the case, and their use appears to be only to give
attachment to the membrane uniting the valves together, and to the
peduncle. The basal margin is internally strengthened by a calcified
rim, more or less developed. The umbones (and primordial valves when
distinguishable,) are seated at the rostral angles; during growth the
basal margin is not added to, and the occludent margin only to small
extent; hence the main growth of the valve is at the upper end, and
along the carina-tergal margin. In _L. fascicularis_, however, the basal
reflexed margin is slightly added to beneath the umbo.

_Terga_,--flat, small compared with the scuta, usually of an irregular
quadrilateral figure, with the two upper or occludent margins very
short, in proportion to the two (carinal and scutal) lower margins; all
the margins are nearly straight. The two occludent margins, generally
meet each other at about right angles, forming a small triangular
projection; in _L. fascicularis_, however, the occludent margin is
formed by a single, slightly curved line. The umbones (and primordial
valves when distinguishable) are not seated at the uppermost point, but
at the angle where the carinal margin unites to the upper of the two
occludent margins: during growth the terga are added to, both on the
occludent and on the scutal margins, and slightly along the carinal
margin; hence their growth is unequally _quaqua-versal_, except at one
angle of the irregular quadrilateral figure.

_Carina._--This is always very narrow and curved, concave within, often
carinated and barbed exteriorly; it extends upwards between the terga
for one half or two thirds of their length: at the lower extremity it
ends (with the exception of _L. fascicularis_), in a small fork (Pl. I,
fig. 1, _a_, _b_) rectangularly inflected and embedded in the membrane,
beneath the basal margin of the scuta. From