d the other valves to an unusually small
degree, broadly oval; basal angle bluntly pointed, apex rounded, blunt;
scutal margin, hollowed out to receive the upper part of the tergal
margin of the scuta; carinal margin curved and protuberant; occludent
margin consists of two short sides at right angles to each other. The
whole valve in length and area is about equal to the scuta; internally,
somewhat concave.

_Carina_, triangular, rather narrow, internally deeply concave, very
slightly curled inwards; basal margin protuberant, with a large central
portion considerably hollowed out.

_Rostrum_, triangular, of nearly the same shape as the carina, but only
one third of its length, internally very slightly concave, and with the
basal margin various, being either truncated or angularly prominent in
the middle.

_Latera._--The upper pair (corresponding to the interval between the
scuta and terga) is only a trifle larger than the latera immediately
beneath; and these only a little larger than those lower down. In the
lowest whorl, the valves are very minute, though still about twice as
large as the scales on the peduncle, and of a different shape from them.
The upper latera (viewed internally) are almost diamond-shaped, owing to
the prominence of the basal margin, but this varies considerably in
degree. The latera in the next whorl are triangular, with the basal
margins protuberant and arched, in a less and less degree in the lower
whorls, until in the lowest, the valves are elongated transversely.

_Microscopical Structure._--A valve placed in acid leaves a thick opaque
mass, formed of three different kinds of tissue, one having a finely
shaded appearance; a second with a largely hexagonal reticulated
structure, and the third thin, transparent, and marked with arborescent
lines, which I imagine to be tubes, as will be hereafter seen in
Lithotrya. Near the exterior surface, there are many tubuli. It appears
to me probable that the strong tendency which the valves in this species
have to disintegrate, is connected with the unusual quantity of
animalized tissue contained by them. Externally the valves are covered
by a strong membrane, either white or yellow, or white streaked with
yellow, and marked by lines of growth, and by longitudinal, sinuous,
little ridges.

_Peduncle_, in the upper part, of rather less diameter than the
capitulum; twice or thrice as long as it; tapering a little downwards;
surface of attachment wide