This is a crested form of the Myrtillocactus geometrizans.

The standard Myrtillocactus geometrizans is a highly branched columnar candelabra like tree cactus that creates a dense growth of stems growing closely together. In nature grows up to 4.5 m tall, with the crown reaching up to 5 m in width..
Flowers: Rather smaller (2.5-3.7 cm) in relation to stem, greenish-white.
Blooming season: March. Starts blooming when it is about 60 cm tall.
Fruit: Very sweet, edible, dark red, oblong fruits 8-20 mm in diameter.
The crests are frequently grafted onto a normal a Myrtillocactus trunk but are easily grown on their own roots. Any normal shoots should be removed to promote the growth of the crest. This cactus tolerates exposure to full sun and generally welcomes moderate watering. They are semi-hardy. Make sure that your Myrtillocacti are not exposed to temperatures below -0°C, or they may die. Nevertheless, it is good advice never let the night-time temperature fall below 10°C. Water regularly in summer, but allow to dry fully before watering again. It needs a well-drained soil mix, with small gravel added to ensure drainage. During the winter months, plants should be rather kept dry, and water is restricted to only enough to keep the stems and branches from shriveling. Since they are big-sized plants need plenty of space for their roots, repotting should be done every other year, or when the plant has outgrown its pot. Exposure: Light shade when young, full sun later.
Propagation: Cuttings in summer (Cuttings will root only in hot weather. Cuttings must be kept very dry to root).

Some of the information in this description has been found at desert-tropicals.com, llifle.com and cactus-art.biz

Bloom Season	March
Botanical Name	Myrtillocactus geometrizans
Dormancy	Winter
Family	Cactaceae
Flower Color	Greenishwhite
Growth Habit	Columnar
Mature Size	4.5 m
Native Area	Indigenous to mexico
Resistance	3.8