_Compartments_: their exterior appearance has been already described. The parietal tubes are not large; they are generally crossed by transverse septa in their uppermost part; but they are sometimes almost solidly filled up by dark shelly layers. The internal surface of the parietes is more or less plainly ribbed; in old specimens, however, it generally becomes smooth. The _radii_ have their septa denticulated on both sides; and they are porose, that is, the interspaces between the septa are not filled up solidly. The radii generally extend from tip to tip of the adjoining compartments, that is, their summits are parallel to the basis; but in three not common varieties, viz., _vars. intermedius_, _occator_, and _d'Orbignii_ (the former of which at least must, without the smallest doubt, be ranked as a mere variety), their summits are oblique. I have occasionally met with specimens of _var. communis_ with oblique radii; and this is not very infrequent in young shells. Exteriorly the radii are generally smooth, but sometimes finely ribbed horizontally, owing to the projection of the septa. The _alæ_ have their sutural edges smooth; their summits are usually parallel to the basis, but they are often much broken; in _var. zebra_, however, in every specimen which I examined, the summits were oblique. The sheath varies much in colour: in _var. occator_, and in a less degree in _var. intermedius_ and _var. d'Orbignii_, the portion lining the rostrum and lateral compartments is much darker than the other parts of the sheath. The _Basis_ generally has a thick, underlying, cancellated layer. Sometimes the basis (Pl. 1, fig. _b_) is irregularly cup-formed. _Mouth_: labrum with four or six minute teeth: mandibles with five graduated teeth; inferior point more or less spinose. Maxillæ, either with or without a small notch, beneath the upper pair of spines; in the lower part there are two spines longer than those immediately above them. _Cirri_, the first pair has the rami unequal, in the proportion of about 19 segments in the longer ramus, to 16 in the shorter. The segments in the latter have their anterior surfaces very protuberant. The second pair is short, with the anterior surfaces of the segments protuberant. On the thorax (Pl. 25, fig. 1), on each side, at the bases of third pair of cirri, there is a projecting membranous plate fringed with fine bristles. The three posterior pairs have their segments shield-shaped in front, generally bearing four pairs of spines, of which the lower pair is minute; between these pairs there are some minute spines. In some young specimens from the Cape of Good Hope, and in _var. concinnus_, I found six pairs of spines on the segments of the posterior cirri.