at between the
tip of the second tooth and inferior angle. Maxillae widely notched, with
the inferior part forming two prominences. Caudal appendages shorter
than, or barely exceeding in length, the pedicels of the sixth cirrus.

Friendly Archipelago, Mus. Paris; Philippine Archipelago, Mus.
Cuming; imbedded in coral rock.

Capitulum rather thick, with the five main valves having their free
apices, diverging and truncated. The upper and old layers of shell do
not here scale off so readily as in many of the foregoing species; and
hence an unusually large proportional length of each valve projects
freely above the sack; and the valves are of unusual thickness. The
capitulum is very nearly as wide at its summit as at its base, owing to
the divergence of the apices of the valves. The scuta and terga are
articulated together by a conspicuous fold, which, when seen from
vertically above, (Pl. IX, fig. 1 _a'_,) appears like a deep
wedge-formed notch in the terga. On the exterior surfaces of the valves,
the teeth on the successive rims are approximate; on the inner surfaces,
the rims are covered by strong yellow membrane, which is generally
fringed with small horny spines.

_Scuta_, exterior surface convex, sub-triangular, with the apex
truncated: seen vertically from above, there is a small rectangular
indentation or fold which receives the projecting scutal margin of the
terga. The inner growing or corium-covered surface (fig. 1 _b_, _b'_) is
triangular, with its tergal margin _largely_ hollowed out. Along the
occludent margin there is a slight ridge, which terminates at the
rostral angle, in both the right and left-hand valves, in a rounded,
knob-like, roughened tooth. The lower part of the tergal margin is
slightly inflected and roughened, where it meets the corresponding lower
part of the scutal margin of the terga. There is a deep pit for the
adductor muscle. The interior surface of the valve above this pit is
faintly-coloured purple. The inner surfaces of both scuta and terga, are
roughened with little points.

_Terga_, seen externally, are almost quadrilateral (owing to the apex
being truncated), with the free margin facing the scutum, arched. Seen
vertically from above, each shows a deep fold, which receives the lower
part of the tergal margin of the scutum. In the foregoing species, a
prominent ridge runs down the exterior surface of the terga from the
apex to the basal angle, against which ridge, the marg