ely downwards, and then both upwards and downwards. The
rostral and carinal latera (with the same exception of _S. villosum_)
have their umbones seated laterally, at opposite ends of the
capitulum,--the umbones of the rostral latera being close to the
rostrum, and those of the carinal pair close to the carina, and
consequently their chief growth is directed towards each other. The
carina in all the species, except _S. villosum_, is either bowed or
angularly bent; in the latter case the lower half is parallel to the
peduncle, and the upper half, extending far up between the terga, is
parallel to their longer axes. In some of the species the carina is
added to almost equally at both ends; in _S. ornatum_ it grows but
little at the upper end, and to a varying degree in different
individuals according to their age; in _S. rutilum_ the umbo is at the
apex, and there is consequently no upward growth; lastly, in _S.
villosum_ the carina widening much from the apex to the basal margin,
grows exclusively downwards, and a portion of the apex projects
freely,--characters all common to the carina in the genus Pollicipes.
The upper latera occur in all the species; in the lower whorl there are
either two or three pair of latera, in the former case the infra-median
pair being absent. The latera differ considerably in shape in the
different species.

The _Peduncle_ is generally rather short, and, with the exception of _S.
Peronii_, is covered with calcified scales. These scales are generally
small, and placed symmetrically in close whorls, in an imbricated order,
with each scale corresponding to the interspace between two scales in
the whorls above and below. In _S. ornatum_, the scales are so wide,
transversely, that there are only four in each whorl. In _S. villosum_,
the scales are spindle-shaped and arranged somewhat irregularly in
transverse rows, not very near to each other. New calcareous scales
originate only round the top of the peduncle, and they continue to grow
only in the few upper whorls; and as the peduncle itself continues to
increase in diameter by the formation of new inner membranous layers and
the disintegration of the old outer layers, the calcareous scales come
in the lower part of the peduncle to stand further and further apart. In
the earliest stage of growth there are no calcareous scales on the
peduncle in _S. vulgare_; they first appear under the carina. Spines are
articulated in great numbers on the surface