ion is hardly visible, and does
not allow of movement; the fissure thus formed runs almost in the line
connecting the umbo and apex, (where in most species a ridge extends,)
but a little on the carinal side of it. The occludent segment is
narrowly bow-shaped, pointed at both ends, with the upper end projecting
slightly beyond the apex of the lateral segment, and with the occludent
margin regularly curved from end to end. The lateral segment is large,
of an oval shape, with a narrow strip cut off on one side. Primordial
valves very plain at the umbones of the lateral segments, but none are
visible on the occludent segments; and this makes me believe that these
two pieces are normally parts of a single valve; having only one
specimen of _P. fissa_, I was not able to make out quite certainly
whether the two segments are continuously united at their umbones by a
non-calcified portion of valve, as is certainly the case with
Dichelaspis. The basal margin of the lateral segment is narrow,
inflected, and blends with the carino-tergal margin; it has an internal,
prominent, basal rim, and towards the occludent margin a large,
prominent, internal tooth. This internal basal rim is not parallel to
the outer basal margin, but rises to a point a little way up the
occludent margin, in the same manner as in _P. eburnea_, but in a lesser
degree; in this latter species the peduncle is internally almost cut off
by the large disc of its carina; here, on the other hand, it is
internally almost cut off by these rims and the two large teeth of the
lateral segments of the scuta.

_Terga_ sub-triangular, short, nearly half as broad as long; three or
four times as wide as the carina, and rather wider than the occludent
segment of the scuta; occludent margin single, arched; carinal margin
slightly arched; basal angle bluntly pointed.

_Carina_ very narrow, much arched, running up just between the basal
ends of the terga; exterior ridge enveloped in membrane; heel blunt,
prominent; internally, not concave, even slightly convex, produced at
the lower end into a very narrow, short, imbedded disc, (or rather
tooth,) which is itself a little curved downwards and blunt at the end.

_Peduncle_ very narrow, about half as long as the capitulum; yellow,
finely beaded, plainly ringed, without spines.

_Mouth._--Labrum, with a row of minute teeth; palpi narrow. Mandibles
with all the lower part narrow; of the four teeth, the second and third
are narrow, t