arying angle,
even up to a right angle,--an external rim or collar being thus formed.
There are no distinct _internal_ teeth, but the basal margin under the
umbones, is more or less distinctly produced into a rounded disc or
projection, which is generally not so much outwardly reflexed as the
rest of the basal margin: there is no distinct internal basal rim. The
primordial valves are generally visible, but they do not lie, as in all
other species, close to the basal margin, but a little above it,--the
lower reflexed portion having been subsequently developed. _Terga_ flat,
with the occludent margin slightly arched, and not, as in the foregoing
species, formed of two sides; apex bent towards the carina; width of the
lower half highly variable, owing to the varying extent to which the
scutal margin is hollowed out; in some specimens, the whole lower half
beneath the apex of the scuta is of nearly the same width throughout; in
other specimens this lower part is spear-shaped. The widest part of the
tergum either equals in width, or is only two-thirds of the width of the
widest part of the carina beneath its umbo. _Carina_ (Pl. I, fig. 6 _a_)
highly variable in shape, with the part above the umbo either
spear-shaped and slightly concave within, or nearly flat and furnished
with a central external ridge; or the upper part (fig. 6 _c_) is of
equal and extreme narrowness throughout, and deeply concave within,
appearing as if only the central ridge had been developed. The part
below the umbo, (answering to the fork in the foregoing species,) is
about one-third of the length of the whole valve, and generally twice as
wide as the upper part, but in the variety with the upper part of the
carina equally narrow throughout, the lower part is thrice as wide as
the upper; the disc, or lower part, is generally slightly concave
within, exteriorly either with or without a central ridge; basal margin
rounded; lateral margin more or less curved, according to the form of
the upper part. The disc is not more deeply imbedded in membrane than is
the upper part of the valve. The heel or umbo is either angular and
prominent, or rounded. In very young specimens the carina is simply
bowed, instead of being rectangularly bent.

_Peduncle_,--short, narrow, being abruptly inflected all round under the
basal edges of the capitulum; lower part of very variable shape, being
often suddenly contracted into a mere thread (fig. 6 _b_), which
sometimes widens