Chromatophores usually single, lying on the ventral connective zone. Mereschkowsky describes nine forms.
Cleve divides the genus into a number of groups as follows:
Amphora proper.—Connective zone not complex; valves with longitudinal lines on the dorsal side; coarsely punctate or costate.
Diplamphora.—Zone complex; otherwise as in Amphora.
Halamphora.—Longitudinal lines absent; frustule elongate, with protracted ends.
Oxyamphora.—Zone complex; longitudinal lines absent; frustule elliptical; valve lunate, with or without a central stauros; striæ punctate.
Amblyamphora.—Zone complex; frustule rectangular; valve lunate; striæ punctate; axial and central areas indistinct.
Psammamphora.—Zone not complex; frustule rectangular; central nodule frequently dilated to a stauros; no axial or central area.
Cymbamphora.—Valve semi-lanceolate; median line straight, approximate to the ventral margin.
Amphora