Thelocactus seeds are medium-sized to large, with the longest axis of 1,5-2,3 mm and a diameter of 1,1-1,7 mm. The hilum-micropylar border is expanded around and constricted above the hilum. Testa cells are smaller near the hilum. Anticlinal boundaries are raised, straight to irregularly curved. Outer periclinal walls are tabular, domed or conical with a smooth or verrucose surface sculpture.