_Genera_--CHTHAMALUS, CHAMÆSIPHO, AND PACHYLASMA.
Fig. 1 _a_, Chthamalus intertextus, shell; 1 _b_, scutum and
tergum, calcified together, with the suture obliterated in
the upper part, internal view of.
2 _a_, Chthamalus scabrosus, shell; 2 _b_, scutum, internal
view of; 2 _c_, tergum, internal view, _p_, plate forming
one side of the cavity in which the depressor muscle is
attached; 2 _d_, tergum, as seen from almost vertically
beneath; _p_, pit for the depressor muscle; _q_, special
pit for the lateral, properly scutal depressor muscle; _r_,
articular furrow.
3 _a_, Chamæsipho columna, shell; A, rostrum; B, lateral
compartment, probably formed by fusion of the
rostro-lateral and lateral compartments; _E_, carina;
3 _b_, 3 _c_ scutum and tergum, internal views of.
4 _a_, Chamæsipho scutelliformis, shell much enlarged, seen
from above; A, rostrum, separated by imperfect sutures from
the (B) lateral compartments, which are formed probably by
the fusion of the rostro-lateral and lateral compartments;
E, carina; B stands opposite one of the inward folds of
the wall, leading into one of the tubular apertures, which
runs (generally with an irregular spiral curve) through
the whole thickness of the shell down to the surface of
attachment. The carina has two of these apertures.
4 _b_, Chamæsipho scutelliformis, another specimen, seen from
the under side, letters of reference the same.
4 _c_, Chamæsipho scutelliformis, rostrum, disarticulated,
enlarged, showing the alæ.
4 _d_, Chamæsipho scutelliformis, scutum and tergum, internal
views of.
5 _a_, Pachylasma giganteum; 5 _b_, rostrum of an extremely
young specimen, much enlarged, showing its compounded
nature, viz., of a true rostrum and rostro-lateral
compartments; 5 _c_, scutum and tergum, external views of;
5 _d_, scutum and tergum, internal views of.