Shell conical, relatively tall, without marginal slit or apical hole; a small internal septum separates the cavity of the shell apex from the rest. Apex forms a small backwardly-pointing vertical coil behind mid-point of shell. Surface marked with a shallow reticulation of radial and concentric ridges. White or grey-brown.
The shell is a little transparent when fresh. Its anterior profile is convex, the lateral profiles straight, the posterior slightly S-shaped. The aperture is oval, the anterior end slightly narrower than the posterior. Shell white and shiny internally. Up to 4 mm long, 3 mm broad, 2.5 mm high. The animal, cream in colour, has much the appearance of a small Patella, but lacks gills and visible eyes. The foot has an oval or shield-shaped sole, and has no operculum. The lateral margins of the dorsal lip are drawn out into short processes lying under the cephalic tentacles.

P. exiguum is not common but has been found in most parts of the British Isles from Shetland and Rockall south to the Celtic Sea; it is absent from the Channel and from most of the North Sea. It does not occur between tidemarks but lives on hard, stony bottoms from 7 to 190 m deep. It is probably a grazing detritus feeder and has probably a brief trochophoral stage in its life history. Outside the British Isles P. exiguum has been recorded from Scandinavian waters and, at greater depths, south to the Azores and Mediterranean.