The philinids usually possess an internal, oval to elongate, whitish and fragile shell. They have a large head shield, often with a groove in the midline. This head shield is broad anteriorly and either tapers abruptly or is truncate posteriorly. The visceral hump behind the head shield is longer, higher, and truncate posteriorly – either plainly squared off or extended into several lobes.
The foot is large though never longer than the body and its lateral margins are raised into well-developed parapodia. Many philinids spend their adult life ploughing through the substrate surfacing only to spawn and so the muscular and often wedge-shaped body is well suited to this burrowing (infaunal) lifestyle. The gill is not visible externally.
All infaunal philinids are white in colour, but those that spend part or all of their life on the surface are pigmented, ranging through to pale yellow or brown to orange sometimes with darker flecks. Two or three large calcareous gizzard plates are sometimes visible internally, but they may be small or even absent. When present these are used to grind up their infaunal prey (various species of bivalves).