the valve, which slips
in the fossil species (C in the woodcut, fig. 1, given in the
Introduction,) I have designated as the intra-parietes. The lower part
of the valve gradually widens from the umbo downwards; internally, the
whole is deeply concave, and continuously curved. The angle varies at
which the upper and lower portions externally meet each other; but is
never less than 135 deg. The upper part of the carina runs up between the
terga for three-quarters of their length; the basal margin does not
extend down low enough to pass between the carinal latera.

_Rostrum_, (fig. 15 _b'_, seen externally, and highly magnified,)
minute, almost hidden by the enveloping membrane and by the small
prominent umbones of the rostral latera; in area equalling about one
fourth of the rostral latera; externally pyramidal, with the upper side
rather longer than the lower; internally slightly concave, square, with
the upper margin and sometimes with the lower margin, slightly hollowed
out. Umbo of growth nearly central.

_Upper Latera_, flat, irregularly oval, with an almost rectangular
shoulder under the basal angle of the terga; in area, about one third
larger than the largest valve of the lower whorl; the exact degree of
elongation of the oval figure varies a little. Umbo seated a little
above the central point.

_Lower Whorl_,--_Rostral Latera_, nearly twice as long as broad, lying
under the basal margins of the scuta: umbo seated over the rostrum;
opposite end, towards which the valve widens either sensibly or but
little, is either square or rounded; in area, less than any of the other
valves, excepting the rostrum; in breadth, equalling either half or one
third of the height of the infra-median latera; growth, directed chiefly
towards the infra-median latera. The freely-projecting umbo is about one
sixth part of the entire length of the valve.

_Infra-median Latera_, rather larger than the carinal latera; their
shape varies from elongated pentagonal with the angles rounded, to oval,
with the longer axis directed upwards. The umbo is seated a little above
the middle of the basal margin, so that there is some little growth
downwards, but the main growth is upwards. The upper point generally
stands a little above that of the carinal latera.

_Carinal Latera_, flat, less in area than the infra-median latera; basal
margin nearly straight; carinal margin slightly hollowed out, terminal
margin arched and protuberant. The umbones