tera_ flat, four-sided, with the basal margin the longest,
and the baso-carinal angle produced.

_Infra-median Latera_ nearly equalling in area the upper latera; not
descending so low down as the rostral and carinal latera; outline of
lower half semi-oval, of upper half rectangular.

_Carinal Latera_ flat, four-sided, with the basal margin the longest,
and slightly protuberant; baso-rostral angle produced; whole valve
larger than the rostral latus, but closely resembling it in form.

_Sub-carina_ minute, not above one third of the size of the rostral
latera, which are the smallest of the other valves; internally deeply
concave; externally solid, pyramidal, standing out beyond the surface of
the carina, with the umbo at the apex.

The umbones of the four pair of latera are seated a little above the
centre in each valve, on the summit of a raised triangular portion; this
arises from the valve at first growing only downwards, and when added to
at the upper end, the new part forms a ledge at a lower level round the
old part, which had already acquired some thickness.

_Peduncle_, short, about half the length of the capitulum; narrow;
thickly clothed with minute, longitudinally elongated, spindle-shaped,
calcareous scales or beads, which project but little.

_Length_ of the capitulum, rather under 3/10ths of an inch.

In a _Young Specimen_, with its capitulum, together with the peduncle,
only 1/10th of an inch long, the scuta, terga, and carina are very large
in proportion to the valves of the lower whorl. The latter project
more, and are externally more pointed, as in the genus Pollicipes. The
rostrum is well developed; the infra-median latera, in proportion, are
the least of all the valves. The carina is straight and pointed, and
not, relatively to the scuta, quite so long. The scuta are rather
broader in proportion to their length, which would naturally follow from
less having been added to their apices,--these valves at first growing
only downwards. The membrane covering and connecting the valves is
furnished with long thin spines.

_Mouth._--Labrum placed far from the adductor scutorum muscle, with the
upper part exceedingly prominent; apparently there are no teeth on the
crest. Palpi blunt.

_Mandibles_, narrow, with four teeth, of which the second is not smaller
than the others; inferior angle sharp and produced, barely pectinated.

_Maxillae._--Under the two or three great upper spines, there is a tuft
of