Discocactus placentiformis (Lehm.) K.Schum.

Description: Discocactus placentiformis is a small flattish globular cactus from Brazil that flowers on warm summer nights. It is one of the most widely distributed and variable species with several unnecessary subspecies described.
Stem: Low, flattened-globular, less than 10 cm tall (without cephalium about 3,5-8 cm), (6)-12-14(-22) cm in diameter light, dark or bluish green, with woolly cephalium.
Ribs: mostly 10-16 low to acute up to 1,5 cm high, not or only slightly tuberculate, distinctly enlarged at the areoles and without acute sinuses between adjacent areoles on the same rib.
Areoles: Round to oval 5-8 on each rib on plants with cephalium.
Cephalium: 3,5 cm tall, 4-6 cm in diameter, with cream-white, greyish-white or greyish-brown wool and black bristles 1,5-2,5 cm long.
Radial spines: 3-5(-9) conspicuous, pale gray, reddish, brown or blackish with a darker tip , becoming grey with age, 1-3 cm long and up to 2 mm thick, often flattened, clawlike, slightly recurved backwards to the plant, lateral spines longer and pointing downwards.
Central spine: In some plants one central spine appears, but is usually absent. It is erect, 1,5-2 cm long.
Flowers: Nocturnal lasting only a single night, and rapidly collapsing into mush the next morning, extremely fragrant, slender funnelform, up to to 7,5 cm long, white arising from the cephalium.
Blooming season: Summer, it needs a good exposure to with some sun to bloom , so it's pretty rare to have blossoms when in cultivation in greenhouses. It is not unusual for this Melocactus to take 7 or more years before it starts blooming, but once it does, it should every year.
Fruit: Club shaped white or cream sometime with a pink tip, up to 3 cm long and 0,8 cm in diameter containing a few black seeds.

http://www.llifle.com/Encyclopedia/CACTI/Family/Cactaceae/1430/Discocactus_placentiformis