internally
deeply concave, widening much from the top to the bottom; basal margin
highly protuberant, with a central portion either truncated and very
slightly hollowed out, or bluntly and rectangularly pointed, with the
apex itself rounded.

_Rostrum_, not one third of the length of the carina, concave,
triangular, with the basal margin slightly protuberant. Of the other
valves, including the sub-carina and sub-rostrum, the shape of their
inner surfaces is sub-triangular, with the basal margin convex;
externally the umbones are pointed, and slightly curled inwards, so as
to overlap each other like tiles: the smaller valves, however, of the
lower whorls (fig. 1 _a_) are more or less transversely elongated, so as
to become almost elliptic instead of triangular. Of the latera, the
upper pair, which corresponds to the interspace between the scuta and
terga, is the largest, but barely exceeds in size the pair answering to
the carinal latera in Scalpellum, which lie between the terga and
carina: the next largest pair is the rostral, or that between the scuta
and rostrum. Some, however, of the lower latera are of nearly equal
size.

_Peduncle_, narrower, but generally longer than the capitulum; upper
part encased with small calcareous scales, with their apices curved
inwards, and overlapping each other. The inner surface of each scale is
triangular, with the basal margin protuberant. The scales continue to
grow or be added to, only in about the ten upper whorls, which form but
a small part of the whole peduncle; in the lower part, the scales become
further and further separated from each other. The surface of
attachment, in full-grown specimens, is broad; but in two very young
specimens, which I removed with great care after the action of potash, I
found the peduncle ending in a filiform prolongation, such as often
occurs in _Scalpellum vulgare_ and in _Lepas fascicularis_. At the
extremity of the pointed peduncle, there were seated the larval
prehensile antennae, of which the following measurements are given to
show how minute they are.

_Inch._
Length, from apex of disc, to the further
edge of the basal articulation 20/6000
Breadth of basal segment, in broadest part 6/6000
Hoof-like disc, length of 6/6000
Ultimate segment, entire length of 6/6000
"