P. arnostiana are green to dark green cacti that are 4 to 6 centimetres (1.6 to 2.4 in) in height, and 4 to 8 centimetres in diameter. There are between 21 and 30 spiralling ribs, with round areoles topped with white wool. Each areole has from one to three central spines which are yellowish brown to yellowish red in color. There are between 12 and 16 radial spines, which curve, and are from straw-yellow to brownish-red in color. The central spines (which are between 5 and 20 millimetres (0.20 and 0.79 in) long) can be difficult to differentiate from the radial spines. The species produces sulphur yellow flowers (which are between 7 and 7.5 centimetres (2.8 and 3.0 in) in diameter), green fruit (which are between 1.6 and 1.8 centimetres (0.63 and 0.71 in) in length by 1.2 to 1.4 centimetres (0.47 to 0.55 in) wide) and cap-shaped black seeds.