_Walls_, moderately strong: inner lamina slightly ribbed: the denticuli on the bases of the parietal longitudinal septa are sharp: I could not see any transverse septa in the parietal tubes. The _radii_ are rather narrow; their summits are remarkably jagged and very oblique; the septa are plainly denticulated on both sides, but chiefly on the lower side; each septum itself, towards the inner lamina of the radius, branches and divides: the interspaces are filled up nearly solidly. The _alæ_ have apparently their summits less oblique than those of the radii: their sutural edges are finely crenated. The lower edge of the sheath is hollow underneath. The _basis_ is flat; it is rather thin, and imperfectly porose; in parts it is not at all porose, in others it is traversed only by very minute pores: there is nevertheless, in some parts, even where the upper layer is not porose, an underlying, cancellated layer. _Animal's body_ unknown. 24. BALANUS CORRUGATUS. Pl. 6, fig. 3 _a_, 3 _b_. _Shell white, longitudinally folded; radii narrow. Scutum internally without an adductor ridge._ _Fossil_, Sub-Appennine formations; Colle in Tuscany; Mus. Greenough. I have seen only two specimens of this species attached to rock, collected by Mr. Greenough, at Colle, and kindly given by him to me. The species comes near to the living _B. crenatus_, also found fossil in deposits of this same age; it differs, however, distinctly from that species, in having its basis permeated by pores, and, in a less degree, in the sutural edges of the radii being more plainly crenated: the opercular valves of the two species closely resemble each other. This may be the _B. stellaris_ of Bronn, but it is futile attempting to identify the species of this genus merely by external characters, even when aided, as in this case, by an excellent drawing of the shell. _General Appearance._--Shell conical, with broad rounded longitudinal folds; orifice of moderate size, oval; radii narrow, with their upper margins oblique; but the summits of both specimens had been much broken. Colour, as it appears, originally white. Basal diameter of largest specimen 3/4 of an inch. _Scuta_, with the upper portion much reflexed; the articular ridge is very prominent, and the articular furrow of great width; when the valve is viewed from the outside the articular ridge is very conspicuous: there is no adductor ridge. _Terga_, with the longitudinal furrow very slight; the spur is from one third to one fourth of the width of the valve, and its basal end is blunt and almost truncated; it stands about half its own width from the basi-scutal angle. Internally, the articular ridge is very prominent, and the articular furrow narrow and deep, extending down the valve in the line of the spur.