_Description._--The _Capitulum_ is much compressed, and generally produced upwards; it is formed of from twelve to fifteen valves, which are rather thin, and with the exception of _S. ornatum_, almost entirely covered by membrane, bearing spines: the valves are seldom locked very closely together. A sub-rostrum exists only in _S. villosum_, which species leads on to Pollicipes; in _S. vulgare_ the rostrum is rudimentary and hidden. The scuta, terga and carina, are much larger than the other valves: these five valves seem to differ essentially from the others in being at first developed under the form of the so-called primordial valves: the other valves commence by a small indistinct brown spot, very different from the hexagonal tissue of the primordial valves: I saw this very clearly in young specimens of _S. vulgare_. At first, the scuta, terga and carina, grow exclusively downwards (and permanently so in most fossil species), and therefore the growth of the scuta and carina is in an absolutely opposite direction to what it is in Lepas, Pæcilasma and Dichelaspis. After a short period the scuta are added to at their upper ends; the portion thus added, stands at a rather lower level, and projects in a rather different direction from the first-formed part of the valve, giving to it, in some respects, the appearance of having been broken and mended. This structure is common to _S. vulgare_, _S. rostratum_ and _S. Peronii_. The upper Latera (except in _S. villosum_) grow in the same manner, namely, at first exclusively downwards, and then both upwards and downwards. The rostral and carinal latera (with the same exception of _S. villosum_) have their umbones seated laterally, at opposite ends of the capitulum,--the umbones of the rostral latera being close to the rostrum, and those of the carinal pair close to the carina, and consequently their chief growth is directed towards each other. The carina in all the species, except _S. villosum_, is either bowed or angularly bent; in the latter case the lower half is parallel to the peduncle, and the upper half, extending far up between the terga, is parallel to their longer axes. In some of the species the carina is added to almost equally at both ends; in _S. ornatum_ it grows but little at the upper end, and to a varying degree in different individuals according to their age; in _S. rutilum_ the umbo is at the apex, and there is consequently no upward growth; lastly, in _S. villosum_ the carina widening much from the apex to the basal margin, grows exclusively downwards, and a portion of the apex projects freely,--characters all common to the carina in the genus Pollicipes. The upper latera occur in all the species; in the lower whorl there are either two or three pair of latera, in the former case the infra-median pair being absent. The latera differ considerably in shape in the different species.