face of the upper free part of the carina. Here the hooked
spines (fig. 4 _d_) are trifid or quadrifid, and are very conspicuous.

_Scuta_, as seen externally, triangular; they overlap half the width of
the terga; on their internal faces (fig. 4 _a_), in the upper projecting
part, there is a strong ridge, against which the scutal margin of the
terga abuts. There is a deep and conspicuous pit for the adductor
muscle.

_Terga_, as seen externally, nearly triangular. The ridge which leads
from the apex to the basal angle, is rounded, central, and extremely
prominent; but does not form a furrow, or include the overlapping margin
of the scuta. The basal angle is narrow, spur-like, and slightly
hollowed out on both margins. The growing corium-covered surface (fig.
4 _b_) is transversely elongated, with the occludent margin rounded, and
the carinal angle much produced, but not forming a roughened knob.

_Carina_ (fig. 4 _d_), concave within, with a slight central ridge in
the upper free portion. The inner growing surface is concave, almost
pentagonal, with a just perceptibly raised central rim in the upper
part, and with two minute prominences on each side, against which the
produced carinal angles of the terga abut.

_Rostrum_, lost.

_Latera_ (fig. 4 _c_), growing surface (or a section parallel to the
growth-layers,) symmetrically oval, more than one third as wide as the
basal margin of the carina. Several zones of growth preserved.

_Peduncle_, lost, but a few scales accidentally adhering to one of the
valves, show that they are crenated in the three or four upper whorls.
No basal calcareous cup was preserved, but by clearing out the base of
one of the holes in the coral, in which a specimen had been imbedded, I
found a little flat disc about the size of a pin's head; it was composed
of two or three layers, and was externally coated by yellow membrane,
including the usual spindle-shaped bodies and tubuli. The cement-ducts
were also discovered after dissolution in acid. So that there could be
no doubt regarding the nature of the little disc.

_Mouth._--Labrum with a row of little blunt teeth.

_Palpi_, blunt, rather expanded at their ends, with the extreme margin
much arched and furnished with two rows of long spines; there is a
fringe of short spines on the straight inner side.

_Mandibles._--There are nine pectinations between the first and second
main teeth, and only two between the second and third teeth; the