en which it extends for three fourths
of their length. The external surface is rounded, except near the umbo,
where the edge is carinated; growth almost equally upwards and
downwards; the parietes and tectum are not separated by ridges.

The _Sub-carina_ lies close under the carina, and is placed almost
transversely to the longer axis of the capitulum; external surface
arched and smooth, the whole having the shape of half of a cone, with
the apex a little curved outwards; seen internally, it may be said to be
formed of two triangular wings placed at right angles to each other;
basal margin straight; in size equalling the carinal latera.

_Rostrum_, lying almost transversely to the longer axis of the
capitulum, under the basal margins of the scuta; in shape (fig. 6 _a_)
closely resembling the sub-carina, but about one third larger than it;
larger also than either the rostral or carinal latera; seen externally,
appears like a half cone; seen internally, is formed of two triangular
wings (with curved edges), placed at right-angles to each other.

_Upper Latera_, internally flat, oblong, twice as long as broad; upper
end square, truncated; upper half rather wider than the lower half;
fully twice as large as either of the lower latera. The basal points
extend below the basal margins of the scuta. The umbo is placed a little
above the centre.

_Rostral Latera_, minute, scarcely exceeding one third of the size of
the carinal latera, and very much less than the rostrum; they are placed
transversely under the basal point of the upper latus, or rather between
it and the baso-lateral angle of the scutum; basal margin, as seen
internally, straight; upper margin arched; rostral angle produced;
internally flat; the whole valve is very thick and solid, so that the
umbo which lies at the rostral end, projects rectangularly outwards.

_Carinal Latera_, oblong, nearly quadrilateral, with the upper angle
produced; placed obliquely, parallel to the lower half of the upper
latera; umbo slightly prominent, seated near the apex, with three
rounded ridges proceeding from it; internal surface very slightly
concave.

_Peduncle and Attachment._--The peduncle is short, not equalling the
capitulum in length. The whole surface is most thickly clothed with
minute spines, which are not visible when the specimen is dry; I think
it probable that they may sometimes all drop off before a new period of
exuviation. The peduncle does not (at least in t