Terga, projecting beyond 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 and flat. Calcareous scales, minute, symmetrically and closely packed together: each scale is much flattened, and its shape, including the imbedded portion, is that of a spear with its point broken off. The basal end of each scale is conically hollow, and from the layers of growth conforming to this hollow, there is a false appearance of an open tube running through the scale.

Attachment.—The surface of attachment is wide: the two cement-ducts, after running down the sides of the peduncle in a sinuous course, within the longitudinal muscles and close outside the ovarian tubes, pass through the corium, and then separately form the most abrupt loops or folds. These are represented in [Pl. IX], [fig. 2], in which a space about 1/10th of an inch square is given, as seen from the outside. At each of the bends, an aperture has been formed through the membrane of the peduncle, and cement poured forth. The manner in which the discs of cement (b) come out of the two ducts (a a), and reach the external surface, is shown in the section, [figure 2 a´]. The two tubes are firmly attached to the older layers of membrane, and are covered by the last-formed layers. In a young specimen, the cement-ducts were a little above 2/2000ths of an inch in diameter, which had increased, in a medium-sized specimen, to 5/2000. The cement-glands are retort-shaped, seated near each other, high up in the peduncle.

Size.—The largest specimen which I have seen, was three inches in length including the peduncle; the capitulum was 9/10ths of an inch long, and one in width.

Young Specimen.—I examined one with a capitulum 18/1000ths of an inch long, measured from the lowest whorl to the tips of the terga; the width was only 13/1000ths of an inch; in old specimens the width of the capitulum is greater than the length. The length of one of the scuta was 14/1000ths of an inch, therefore, greater than the width of the entire capitulum, which is not the case with mature specimens. Besides the scuta and terga, the carina and rostrum, and three pair of large latera, there was a lower whorl formed of ten or twelve valves, giving altogether to the capitulum of this very small specimen, either twenty-two or twenty-four valves.

Shape of Body, Sack, Colours, &c.—From the position of the orifice of the capitulum, the animal’s body is suspended to the scuta in a more transverse direction than is usual. The prosoma is well-developed, and is distinctly separated from the three posterior thoracic segments, by a band of thin membrane. The tunic of the basal part of the sack, where it enters the peduncle in a blunt point, is thickened and covered with roughened rounded papillæ. The corium of the sack under the valves, is coloured (after spirits) so dark a brown as to be nearly black; the cirri and trophi are similar, but with a tinge of greenish-purple.