The pedal slit in the mantle occupies the whole length of the latter, and is confluent with the anterior or also with the posterior respiratory aperture; so that only one or even no aperture is left, and this when present is besides never elongated in a tubular form; the retractor muscles are therefore wanting also, and the shell simply exhibits a discoid groove for the mantle. The occlusor muscles approximate and unite at their terminations. Both usually coalesce into one.
Fam. 4. Typical Mussels—Mytilaceæ.
Occlusor muscles separate, and mantle slit to such an extent, that only the posterior respiratory aperture remains; foot mostly tongue-shaped or coniform, with occasionally a byssus.
Here belong the fresh-water Mussels and the Mytili proper, of which last many bore into rocks. They typify the Capulidæ and Brachiopoda.
Fam. 5. Snail-Mussels—Arcaceæ.
Have two separate occlusor muscles and a perfectly separate mantle without respiratory aperture; the foot small, mostly cartilaginous. Here belong the Arcæ, Aviculæ, and Pearl-mussels.
They typify the Trochidæ and Pteropoda.
Fam. 6. Kracken-Mussels—Ostracea.
Only one occlusor muscle, mantle entirely open or slit, so that both respiratory apertures are only oblique incisions therein; foot very small, frequently furnished with a byssus. They typify the Buccinidæ and Cuttle-fish.
Fifth Class.