General Appearance.—Shell conical, smooth, with a tinge of orange colour; orifice large, deeply notched, sub-triangular. Viewed externally, the compartments in appearance are only four; but on close examination, the lateral compartments are seen to be divided by a very fine fissure into two nearly equal compartments. The rostrum is broad and flat, and when carefully examined, it also is seen to be divided by two fine fissures into three compartments; of these the middle one, or true rostrum, is a very little broader than the rostro-lateral compartment on each side; hence, on careful examination, the shell is found to consist of eight compartments. The carina is much compressed. The radii (in my one specimen) are not developed. The carina, and the two lateral compartments alone, have alæ; for the carino-lateral compartments are too closely joined to the lateral compartments, and the true rostrum is too closely joined to the rostro-lateral compartments, to have their alæ developed. The four alæ which are developed, are very broad, widely exposed, and marked externally by lines of growth. The basal diameter of my one, apparently old specimen, is one inch.
Structure of Shell, &c.—The inner surface (fig. [1 b]) of the rather thick parietes is smooth, except close to the base, where it is roughened by a few irregular points. The sheath descends low down, and has its lower edge slightly free. The sutures are much plainer on the internal than on the external surface, and can be here plainly seen to be eight in number. That portion of the carino-lateral compartment, which helps to form the sheath, is narrow, though the whole compartment is of very nearly equal width with the lateral compartment. It is a singular fact, that no portion of the rostro-lateral compartment helps to form the sheath; for the alæ of the lateral compartments, overlap the whole upper part of the rostro-lateral compartments, and abut against the true rostrum. Hence, when the sheath alone is examined, the number of the compartments appears only six. In a section the true rostrum can be seen to underlie the rostro-lateral compartments, and thus exhibits vestiges of alæ. The fact of the rudimentary rostro-lateral compartments not forming a part of the sheath offers a marked difference from [Chelonobia], which otherwise has this part of the shell very similarly constructed. The diametric growth of the shell, which seems to be considerable, is effected by the four large alæ of the carina and of the lateral compartment on each side. The sutural edges of the alæ are added to in a regularly inclined line down to the basis. The basis is calcareous, and not very thin.
Both opercular valves (fig. [1 c], [1 d]) closely resemble those of the last species.
The Scuta are remarkably narrow and elongated: the external surface is slightly furrowed longitudinally, the prominent lines of growth are much wrinkled. The articular ridge is blunt, and not very prominent. The upper part of the valve is reflexed, and a considerable portion must have projected freely. The Terga are nearly twice as broad as the scuta: the carinal half of the valve is very smooth, and is coloured beautiful reddish-orange. The spur is tolerably distinct. The articular ridge is not very prominent.
The Mouth resembles that of the last species. The mandibles have three sharp teeth, with the inferior part narrow and pectinated with long spines: the edge, also, between the upper main teeth, is pectinated with short spines, which latter often have their summits crenated. The maxillæ are notched; the inferior corner is produced into a small step-formed projection. The outer maxillæ are more pointed than is usual.
The Cirri most closely resemble in every detail those of [P. giganteum]; I can point out only one slight difference, namely, that the lower segments, in the posterior ramus of the third cirrus, which are thickly clothed with spines, are more numerous in proportion to the upper segments with the spines arranged in regular pairs (being as 4 to 14), than in [P. giganteum], in which only one fifth (or 4 to 20) are so clothed.
Caudal Appendages.—These are very small, not being more than one fifth of the length of the pedicel of the sixth cirrus: their segments are indistinct, and they support a very few coarse spines. The rami of the sixth cirrus, in the one specimen, had twenty-five segments, whilst each caudal appendage had only five. Hence the caudal appendages are far less developed than in [P. giganteum].