_Scuta._--In three of the species the two segments, named the occludent and basal, appear like separate valves, but these, by dissection, can be most distinctly seen to be united at the rostral angle. The primordial valve, formed of the usual hexagonal tissue, is elliptic, elongated, and placed in the direction of the occludent segment; calcification commences at its upper point, so as to form the occludent segment, and afterwards at its lower point, but rectangularly outwards, to form the basal segment; in the minute space between these two points of the primordial valve, there is, in four of the species, no calcification; so that the two segments are united by what may be called a flexible hinge; in _D. orthogonia_ the two calcareous segments are absolutely continuous. The occludent segment is longer than the basal segment; it either runs close along the orifice, or in the upper part bends inwards; both segments are narrow, except in _D. Warwickii_, in which the basal segment is moderately broad; the two segments are placed at an angle, varying from 45° to 90°, to each other. The capitulum generally extends for a little space beneath the basal segments of the scuta, where it contracts to form the peduncle. The _Terga_ present singular differences in shape, and are described under the head of each species; scarcely any point can be predicated of them in common, except that they are flat and thin. The _Carina_ is much bowed, narrow, and internally either slightly concave or convex and solid; the upper end extends far up between the terga; the lower end is formed by a rectangularly inflected, imbedded, triangular or oblong disc, deeply notched at the end, or as in _H. Lowei_, of a fork, the base, however, of which is wider than the rest of the carina, so as to present some traces of the disc-like structure of the other two species; or lastly, as in _D. orthogonia_, it terminates in a crescent-formed cup. _Peduncle._--This is narrow, compressed, and about as long, or twice as long, as the capitulum; in _D. Warwickii_ it is studded with minute beads of yellowish chitine. _Size._--Small, with a capitulum scarcely exceeding a quarter of a inch in length. _Filamentary Appendages._--None. There are two small ovigerous fræna, which, in _D. Warwickii_, had the glands collected in seven or eight little groups on their margins. _Mouth._--Labrum highly bullate, with small teeth on the crest; palpi small, not thickly covered with spines. _Mandibles_ narrow, with three or four teeth. Maxillæ small, with a notch beneath the two or three great upper spines; lower part bearing only a few pair of spines, generally not projecting, but in _D. orthogonia_ largely projecting. Outer maxillæ, with their inner edges continuously covered with bristles.