Scuta, externally very convex, with the growth-ridges extremely prominent; basal margin sinuous, the middle portion being prominent; this is best seen in young specimens (Pl. [3], fig. [2 d]). Internally, the articular ridge is broad and reflexed. The adductor ridge in the upper part is almost confluent with the articular ridge; it runs down to the most prominent point of the basal margin; in young specimens it is sharp and prominent; in old specimens it is very blunt and little prominent. There is a rather deep hollow for the lateral depressor muscle. In young specimens there is a small, depending, blunt tooth at the basi-tergal angle, which helps to make the basal margin more deeply sinuous.
Terga, with the longitudinal furrow closed, except on the spur itself, where it is open. The spur is moderately long and broad, but varies in breadth; it is placed at rather less than its own width from the basi-scutal angle; its lower end is obliquely rounded; the basal margin on the opposite sides of the spur, together form a nearly straight line. The whole valve is rather broad. The crests for the tergal depressores are barely developed.
The Compartments have rather large parietal tubes; the septa are coarsely denticulated at their bases; the internal lamina is smooth, except close to the basis. The radii have their summits parallel to the basis; their sutural edges are formed of rather thick septa, which stand at an unusual distance apart from each other, and have perfectly symmetrical, minute denticuli on each side. The interspaces between the septa are filled up solidly to within a short distance of the surface; but yet not so completely as in the following species, and as in those in the succeeding sections of the genus; this is what might have been expected from the close affinity of [B. stultus] to [B. Ajax], in which latter the radii are still permeated by pores, though smaller than is general in the species of our first section (A). The alæ have their summits extremely oblique, and their sutural edges, I believe, smooth. Basis porose, with an underlying, finely-cancellated layer.
Mouth: labrum with six small teeth; mandibles with the 3d tooth blunt; the 4th minute, and the 5th almost confluent with the inferior angle. Maxillæ with the edge straight and simple. Cirri partly destroyed; on each segment of the sixth pair there were five pairs of spines.
10. [BALANUS] CALCEOLUS. Pl. [3], fig. [3 a]-[3 e].
BALANUS CALCEOLUS KERATOPHYTO INVOLUTUS (?) Ellis. Phil. Trans., vol. 50 (1758), Tab. 34, fig. 19.
LEPAS CALCEOLUS (?) Pallas. Elench. Zooph., p. 198, (sine descript.) (1766).
CONOPEA OVATA (?) J. E. Gray. Annals of Philosophy, vol. x, 1825.
Parietes and basis porose. Scutum with the pit for the lateral depressor muscle small and deep.