PYRGOMA CANCELLATUM. _Leach_ (!). Encyclop. Brit., Supplement, vol.
3, Pl. 57, 1824.
---- LOBATA. _J. E. Gray_ (!). Annals of Philosophy, (new series),
vol. 10, 1825.
CREUSIA RAYONNANTE. _De Blainville._ Dict. Sc. Nat. (sine descript.),
Pl. 116, fig. 7, _a_, _b_.
_Shell with the circumference generally lobed: scutum elongated, with
the adductor ridge descending far below the basal margin, and produced
at the rostral end into a square point; tergum with the spur four times
as long as the upper part of the valve._
_Hab._--Imbedded in a Gemmipora, probably from the West Indies;[116]
Mus. Brit. and Cuming.
[116] In this case I am again indebted to Mr. Dana for naming the
coral; he informs me that the genus is found in the Pacific and
East Indies; the specimen sent he believes is the _G. cinerascens_.
_Appearance and Structure of Shell._--Shell nearly flat, sometimes
tinted dull dirty purple, with the surface marked by slight, broad,
approximate ridges, the ends of which form considerable projections,
giving to the shell, when not too much encrusted by coral, a lobed
border. In young specimens (.15 of an inch in diameter) some of
the points projected half as much as the semi-diameter of the
shell, giving it a radiating appearance. The orifice is oval and
rather small, but of variable size. The shell is thick, and, near
the outer lamina, is penetrated by pores: the internal surface is
smooth. The sheath (fig. 5 _b_) extends down but a short distance
from the orifice; it is closely attached to the walls: the lines of
growth, at a point on each side, bend a little downwards (instead of
upwards, as in the two following species), and hence the lower edge
of the sheath irregularly projects downwards on each side. The basal
cup, internally, is plicated, the hollows corresponding with the
projecting, longitudinal, parietal septa, which form the lobed border
of the shell. The largest specimen which I have seen, was rather
above .4 of an inch in diameter.
The _Scuta_ and _Terga_ are not calcified together: they are both
much elongated. _Scutum_ (fig. 5 _c_, 5 _e_).--For the first time in
the genus, or indeed in the family, this valve presents a remarkable
character in the adductor ridge being immensely developed, so as to
project far below the ordinary basal margin. At the rostral end, it
at first appears to project even more than it really does, for the
toothed occludent margin is in fact a prolongation of the true valve,
as distinct from the adductor plate. Excluding this very narrow,
prolonged, occludent margin, the adductor plate projects for a
length equalling the rest of the valve. Along the tergal margin, the
adductor plate is united to the articular ridge; and at the rostral
end, it is produced into a square tooth, whence a square-edged ridge
extends on the surface of the plate upwards to the ordinary basal
margin of the valve. The exact shape of this adductor plate varies
a little, as does the degree to which it is closely attached to the
ordinary basal edge of the valve. The valve, as distinct from the
adductor plate, is narrow, with the basal margin regularly curved.
The articular ridge is very prominent.
_Terga_ extremely narrow, linear, consisting in chief part of the
spur, which is fully four fifths of the entire length. Externally
(fig. 5 _d_) the valve is furrowed, with the edges more or less
folded in along the spur. The upper or ordinary part of the valve is
about one third wider than the spur. The basi-carinal angle is sharp,
owing to the basal margin being a little hollowed out. A special
plate of shell (fig. 5 _f_), hollow under its basal edge, runs from
the carinal margin to the articular ridge, which latter is situated
in the middle of the valve, and against which the articular ridge of
the scutum abuts. The spur is central; its end is bluntly pointed.
The total length of the tergum rather exceeds that of the scutum, the
produced adductor ridge being included in the latter.
_Affinities._--Under _P. grande_ I shall make a few remarks, showing
that in several characters _P. cancellatum_ and _grande_ are at
opposite ends of a short series, with _P. conjugatum_ intermediate
between them.