6. CYMBA. Upper edge of the aperture separated from the body whorl by a flat disc; apex mammillated, irregular; folds on the columella. Fig. 434. 7. VOLVARIA. Cylindrical; aperture long, narrow; folds on the columella; spire hidden. Fig. 439. COLUS. Humphrey. FUSUS, Lam. COMPLANARIA. Sw. A subgenus of ALASMODON (Unio), thus described, "shell winged; the valves connate; the bosses very small and depressed; cardinal teeth two or three; lateral teeth represented by irregular grooves. C. gigas (Unio), Sow. Man. fig. 141. Alasmodon complanatus, Say. C. rugosa, Sw." COMPRESSED. Pressed together, or flattened. The application is the same as in common use. A Patella may be described as a vertically compressed cone. A Ranella, on account of the two rows of varices skirting the whorls, appears, as it were, laterally compressed. A bivalve shell is said to be compressed when it is flat, that is, when but a small cavity is left in the deepest part when the valves are closed. Perhaps the Placuna placenta, fig. 184, is the most remarkable instance of this. CONCAMERATIONS. (_Con_, with, _camera_, a chamber.) A series of Chambers joining each other, as in Nautilus, Spirula, &c. CONCENTRIC. A term applied to the direction taken by the lines of growth in spiral and other shells, (_longitudinal_ of some authors.) Every fresh layer of shelly matter forms a new circle round an imaginary line, drawn through the centre of the spiral cone, down from the nucleus. When the edges of the successive layers are marked by any external characters, the shell is said to be concentrically striated, banded, grooved, costated, &c. A fine illustration of the latter is to be seen in the Scalaria or Wentletrap, fig. 351, Lines, bands, ribs, &c. in the opposite direction, (_transverse_ of some authors,) are "radiating" in bivalves, as the ribs of Cardium, fig. 123, and "spiral" in univalves, that is, following the direction of the whorls, as the bands of colour in Pyramidella, fig. 342.