pparently first shadow forth the posterior or carinal,
transversely-striated, opercular muscles of sessile cirripedes.

_Basal Calcareous Cup or Discs._--I have seen this part in all the
species, except _L. Valentiana_, and in this it probably occurs,
considering its very close alliance with _L. truncata_. The size, form,
and conditions of the cup or disc varies infinitely according to the
age, size, and position of the individual specimen. We will commence
with a full-sized animal, which has ceased to burrow downwards into the
rock, in which case the discs usually grow into a cup, and become
largely developed. In _L. dorsalis_ alone, I have seen many specimens,
so that the following description and remarks, though applicable I
believe to all the species, are drawn up from that alone. The cup (Pl.
VIII, fig. 1 _a'_, 1 _c'_) is hardly ever regular in outline, and is
either slightly or very deeply concave; I have seen one, half an inch in
diameter; it is formed of several thick layers of dirty white,
translucent, calcareous matter, with sinuous margins; externally the
surface is very irregular, and is coated by yellow membrane presently to
be described. The innermost and last-formed layer sometimes covers the
whole inside of the cup, and extends a little beyond its margin all
round; but more generally it projects beyond only one side, leaving the
other sides deserted. I have seen a _single_ new layer extending beyond
the underlying old layers, as much as one sixth of an inch; and again I
have seen a part of the cup, as much as a quarter of an inch in width,
deserted and covered with serpulae. So irregular, however, is the growth,
that after a period an old deserted portion will occasionally be again
covered by a new layer, though of course without organic adhesion. Again
it sometimes happens that the last-formed layer, remaining central, is
very much less than the older layers; in one such instance the innermost
and last-formed layer (fig. 1 _a'_) had a diameter of only a quarter of
that of the whole cup, in the middle of which it was placed; the cup
thus tends to become filled up in the middle. The cup, in its fully
developed condition, is seated at the very bottom of the cavity in the
rock. From the aggregate thickness of the several component layers
forming the cup, the old and mature animal rises a little in its burrow;
for instance, the bottom of the cup in one specimen which I measured,
was 4/10ths of an inch in thic