The _alæ_ (so called by Dr. Gray) are overlapped by the radii and by part of the walls; they usually extend only about half way down the compartment (_a_ fig. 3, 4, 1); their summits are either parallel to the basis or oblique. The alæ of the several compartments, together with the internal, upper, thickened surfaces of the walls, against a shoulder of which the sutural edges of the alæ abut, have been called (by Dr. Gray) the _sheath_ (_vagina_). The upper and greater portion of the sheath is marked by transverse lines, caused by the exuviation of the _opercular membrane_, as that membrane may be called, which unites the operculum all round to the sheath, or upper internal surface of the shell. The _carina_ has always two _alæ_, as in fig. 4. The _carino-lateral_ and _lateral compartments_ have always an _ala_ on one (the rostral) side, and a _radius_ on the other (the carinal) side, as in fig. 3. The _rostro-lateral compartment_ (when present) has always _radii_ on both sides, as in fig. 2. The _rostrum_ has normally _alæ_ on both sides, as in fig. 4, but very often from fusion with the rostro-lateral compartments on both sides, it has _radii_ on both sides, as in fig. 2. The walls of the shell, the basis, and the radii, are in very many cases composed of an _outer_ and _inner lamina_, united together by _longitudinal septa_; a set of tubes or pores being thus formed. The points of the longitudinal septa generally project beyond the laminæ, and are denticulated on both sides (see woodcut, further on;) the septa are sometimes branched, several irregular rows of pores between the two laminæ being then formed (see Pl. 7, fig. 3 _b_, and Pl. 10, fig. 1 _g_, 1 _h_). _Operculum_, or _opercular valves_.--These consist of a pair of scuta and a pair of terga. They are joined to the sheath of the shell by the _opercular membrane_.