[117] Dr. Gray thinks this is the _Pyrgoma stellata_, of Chenu,
('Illust. Conch.'); it may be so; but the figure given of the
shell will do equally well or rather better for the _Pyrgomum
dentatum_ of this work, and for some varieties of _P. crenatum_.
Without a careful description of the opercular valves, it is really
impossible to recognise, with any approach to certainty, the
species of this genus.
_Shell nearly flat with approximate radiating ridges: scutum and tergum
calcified together without any suture: scutum with the adductor ridge
descending below the basal margin, and produced at the rostral end
into a point: tergum with the spur about as large as the upper part of
valve._
_Hab._--Red Sea; Brit. Mus. and Cuming.
_Appearance and Structure of Shell._--Shell white, or with a tinge of
purple; nearly flat, with moderately prominent, narrow, approximate
ridges, radiating from the orifice, which is oval, rather narrow,
and not very small. In the largest specimens the ridges are less
prominent than in the figure given. The walls are thick, and not at
all porose: the sheath extends down almost to the base of the shell,
and its lower edge is closely attached to the walls: on each side,
towards the carina, there is a trace of a suture, and the lines of
growth on the sheath are here a little upturned. The basis is deeply
imbedded and internally furrowed; the calcareous layer forming it, is
thin. The largest specimen was .4 of an inch in the longer diameter.
The _Scuta_ and _Terga_ (fig. 7 _b_, 7 _c_), in all the specimens
which I have seen, are calcified together, with no trace of a
suture. There is, however, a slight furrow, which, I believe, marks
the normal line of separation between the two valves; and in the
following description this is assumed to be the case. The _Scutum_
has the adductor ridge greatly developed, so as to project below
the ordinary basal margin to a distance as great as the height of
the valve. At the rostral end, this adductor ridge or plate is
produced into a point; and at the tergal end, it is blended with the
articular ridge, and united to the inner face of the tergum. That
portion of the scutum which corresponds with the valve in ordinary
cases, and alone is externally visible as long as the operculum is
united by the opercular membrane to the sheath, is narrow, with the
basal margin considerably hollowed out: the occludent edge is formed
into thick teeth. The _Tergum_ is elongated, rather exceeding in
length the scutum, the latter being measured from the apex to the
rostral projection of the adductor plate. The surface of the valve is
depressed in the line of the spur, with the basal end of the latter
bluntly pointed. A very slight flexure (fig. 7 _c_) on the basal
margin indicates where we may believe the spur to commence, showing
that it rather exceeds in length the whole upper part of the valve.
The lines of growth obscurely indicate a tendency to the formation of
a slight "occludent ledge" along the carinal margin. Traces are just
visible of crests for the attachment of the tergal depressor muscles.