eum.[31]

_General Appearance._--Capitulum highly bullate, or thick. Valves rather
thick, opaque, either pale or dark flesh-red, smooth, yet rather plainly
striated from the umbones. There are a few very minute spines on the
membranous borders of the valves.

_Scuta_ highly convex, broadly oval, apex broad rounded; basal margin
narrow, much curved; no internal, umbonal teeth; basal internal rim
strong, running up part of the occludent margin. A slightly prominent
ridge, either rounded or angular, but in one specimen a narrow depressed
fissure-like line, runs parallel to the occludent margin and ends near
the apex in a slight notch; this fact is of interest in relation to the
structure of the scuta in _P. eburnea_ and _P. fissa_. The scuta are
either equally or very unequally convex; in the latter case, the
occludent margin of one valve is curled, so that its umbo is not quite
medial.

[31] It is stated, in 'Zoolog. Proc.,' (1848, p. 44,) that this
species was attached to a gorgonia, from Madeira; I cannot but
suspect that there has been some confusion with the _Oxynaspis
celata_ from Madeira, which is thus attached.

_Terga_, minute, almost rudimentary, scarcely broader than the carina,
and half as long as the chord of its arc; carinal margin slightly
curved; scutal margin straight, with a slight prominence fitting into a
notch in the scuta; basal end bluntly pointed.

_Carina_, (fig. 3, _a_) rather shorter than the scuta, extending up only
to the basal ends of the terga; moderately curved; apex moderately
sharp; middle part broadest, externally carinated; internally not
concave, with the inner lamina of shell, at the basal end, produced into
a very small oblong disc or tooth, which is only as wide as the
narrowest upper part of the valve. The exterior keel does not extend on
to this disc, which is slightly constricted at its origin.

_Peduncle_ very short, narrow, ringed, and apparently without spines.

_Size._--Capitulum four tenths of an inch long.

The following parts of the animal are described from some small and not
well preserved specimens from Madeira, which I owe to the kindness of
Mr. Lowe.

_Mouth._--Labrum highly bullate in the upper part, with large, inwardly
pointed, unequal teeth. Mandibles, with four large, pointed, equal-sized
teeth, with the inferior angle very narrow, acuminated like a single
spine. Maxillae, with three (?) large upper spines, of which the middle
one is extreme