_Scuta_ (Pl. 2, fig. 2 _a_, 2 _c_) very smooth, with the growth-ridges very little prominent, sometimes there are obscure traces of longitudinal striæ; surface covered by an unusually thick and persistent yellow membrane: valve narrow, with the upper part commonly reflexed: the basal margin forms, with the occludent margin, a smaller angle than is usual: the tergal margin of the valve is rectangularly inflected, instead of being, as is usual, merely bowed inwards. Internally, the articular ridge is rather prominent. The depth of the _slight_ pit for the lateral depressor muscle is variable; it sometimes includes a minute, central, longitudinal ridge. _Terga_ (2 _b_, 2 _d_): the longitudinal furrow is deep, with the sides folded in; the spur is placed at about its own width from the basi-scutal angle; it is moderately long, with its lower end obliquely rounded off; but the length, breadth, and precise outline of the lower end varies a little. The basal margin on the opposite sides of the spur, forms a nearly straight line, but with the portion on the carinal side very slightly hollowed out. Crests for the depressor muscle are well developed. _Compartments._--The radii and alæ always have their summits oblique: the sutural edges of the radii are deeply penetrated by pores between the strongly denticulated septa: the sutural edges of the alæ are quite smooth: the tubes in the parietes are crossed in the upper part of the shell by septa. _Basis_ tubular, with an underlying cancellated mass. _Mouth._--Labrum with the teeth either absent or very small: mandibles with the fourth and fifth teeth rudimentary: maxillæ with a small notch under the two upper spines; near the lower angle, two spines, one beneath the other, are larger even than the upper pair; beneath the lower pair, there is a tuft of fine spines. _Cirri_, segments protuberant in one ramus of the first cirrus and in both rami of the second cirrus; posterior cirri with the segments short and broad, each bearing three pairs of spines, with a small intermediate tuft. _Affinities._--This species in all essential respects comes very near to the three last varieties of _B. tintinnabulum_, which have the orifices of their shells toothed. The smoothness of the scutum, with its persistent epidermis,--its peculiar shape,--its small and not reflexed articular ridge,--together with the crests on the tergum for the depressor muscles, are sufficient diagnostic characters. Even in general habit and tint of colour, this species has a different aspect from _B. tintinnabulum_. In some respects _B. tulipiformis_ leads into the species included in the third section of the genus. 3. BALANUS PSITTACUS. Pl. 2, fig. 3 _a_-3 _d_. LEPAS PSITTACUS. _Molina._ Hist. Nat. Chile (1788), vol. i, p. 223. BALANUS PICOS. _Lesson._ Zoolog. Voyage de la Coquille (1829).