The scuta are brought together by a short, strong, straight, adductor muscle (Pl. 25, fig. 1, _a_); its attachment leaves (with very few exceptions, as in Tubicinella) a rounded impression, or even pit, on the under side of the valve in its upper part. This pit is frequently bounded, on its lower side, by a sharp ridge, which, though not in actual connexion with the adductor muscle, I have, for convenience sake, called the adductor ridge; it serves apparently to give support to the animal's body; in some few cases (as in _B. psittacus_, Pl. 2, fig. 3 _c_) it is confluent at its upper end with the articular ridge, and converts the whole basi-tergal corner of the valve into a deep cavity. In some of the species of Pyrgoma (Pl. 12, fig. 5 _c_, 7 _b_), and in some varieties of Creusia, this adductor ridge is enormously developed, so as to depend far beneath the true basal margin, or that to which the opercular membrane is attached. At the basi-tergal corner of the valve, there is generally a small pit or impression, and sometimes distinct crests, for the attachment of the lateral depressor muscle. At the rostral end there is, also, a small cavity formed by the overfolding of the occludent margin (rarely furnished with crests) for the attachment of the rostral depressor muscle. In the Terga, at the basi-carinal corner, there are usually crests, though sometimes feebly developed, for the attachment of the tergal depressor muscle. But in Chelonobia, Coronula, Tubicinella, Platylepas, and in some other cases, there are no crests. The crests, when well developed, are furnished with rectangular sub-crests or denticuli on both sides; in fact they resemble, and are probably homologous with, the denticulated ribs or septa in the parietes, radii, and basis. Altogether the scuta and terga are attached, as far as muscles are concerned, to the shell and sack, by three longitudinal pairs. _Growth of the Whole Shell, and Its Microscopical Structure._