Valves at the larger end of the size spectrum of P. brevistriata are lanceolate with rostrate ends. Smaller specimens are elliptical with rounded ends. The axial area is broad and lanceolate. Striae restricted to the valve margin. Striae are distinct, composed of wide, round to oval areolae. Spines are positioned on the valve mantle, in line with the striae. The spines are solid and spatulate.