Phragmoceratids are characterized by generally compressed, upwardly curved endogastric shells with slit-like apertures for egress and ventral siphuncles with broadly expanded segments, thick connecting rings, and small to vestigial bullettes.

Phragmoceratids are morphologically similar in regards to their constricted apertures to the more cylindrical and orthoconic Mandaloceratidae, also discosorids, and to the oncocerid Hemiphragmoceratidae (Sweet 1964); both which also come from the Silurian.

The family has its beginning with the Middle Silurian Protophragmoceras, known from a gradually and evenly expanding endogastric shell with an unrestricted aperture, like that of its cyrtogomphoceratid ancestors, deep sharp hyponomic sinus for the water-jet funnel, and ventral siphuncle, phragmoceratid in character.

Phragmoceras, type genus, is known by its moderately large, strongly curved, rapidly enlarging, endogastric and compressed shell with a vertically constricted aperture that opens up at either end.

Sthenoceras has a fairly large, smooth endogastric shell, more strongly curved in the early stages, aperture that is only slightly contracted, and siphuncle with variably expanded segments and small bulletes.