e terga and carina. There are eight
valves, of which the scuta, terga, and carina are large; the rostrum and
a pair of latera are very small and often rudimentary. These three
latter valves are essentially distinguished from the scales of the
peduncle, the upper ones of which they sometimes hardly exceed in size,
by not being moulted at each period of exuviation. The latera overlie
the carinal half of the terga; I presume that they are homologous with
the carinal latera in Scalpellum. Each successive layer of shell forming
the valves is thick, and extends over nearly the whole inner surface;
hence the carina and terga, and to a certain extent the scuta, either
actually do project freely much beyond the sack, or would have done so,
had not their upper ends been removed; for the upper and old layers of
shell, in most of the species, either scale off or disintegrate and wear
away. A rectangularly projecting rim, serrated by small teeth, is formed
at the bottom of each fresh layer of growth, along the external surfaces
of each valve (see upper part of fig. 1 _b'_ Pl. VIII.) This structure,
as well as that of the crenated scales on the peduncle, is important,
for by this means the animal, as we shall presently see, forms and
enlarges the cavity in the rock or shell in which it is imbedded.

The scutum overlaps either about one third or even one half of the
entire width of the tergum, and abuts against a prominent longitudinal
ridge on its exterior surface. In _L. truncata_ and _L. Valentiana_,
this ridge on the tergum being folded over towards the scutum, forms a
conspicuous furrow, receiving the tergal margin of the latter. In _L.
Valentiana_, there is a second furrow on the carinal side of the tergum,
receiving the upper end of the corium-covered or growing surface of the
carina. Besides these provisions for holding together the valves, there
are, apparently, others for a similar purpose; thus in each scutum,
under the rostral angle, there is a roughened knob-like tooth, which
touches the under side of the little rostrum, and no doubt serves to
give attachment to the membrane uniting the three valves together. In
some species, the adjoining basal margins of the scuta and terga, where
touching each other, are inflected and roughened; again in _L.
Rhodiopus_, the carinal angles of the terga are produced into points,
and in _L. truncata_ and _L. Valentiana_ into prominent roughened knobs,
which touch two corresponding small kn