Schlumbergera truncata (Haworth) Moran in Gentes Herb. 8: 329 (1953). 

Desc from B&R 1923

Plants - joints dark glossy green, about 3 cm. long, sharply serrate, with two prominent teeth at otherwise truncate apex; 
Terminal areole - broad and thin, filled  with brown wool and bristles; 
Flowers - 6 to 7 cm long: tube 2 cm. long; 
Inner perianth-segments - scarlet to white, oblong, obtuse to acute, reflexed; 
Filaments -  white; 
Style -  purple throughout; 
Fruit-  obovoid, 1.5 to 2 cm. long.
Type locality: Brazil.

This species has been cultivated widely for many years under various names. It was intro­duced into cultivation about 1818 and, according to Edwards, flowered first in England in 1822 and has since been a great favorite as a household plant, blooming freely about the end of the year, hence the name Christmas cactus. It is also called crab cactus and ringent-flowered cactus.    

Notes from Bradleya 13
DISTRIBUTION. Brazil (Rio de Janeiro): Serra do Mar including the Serra dos Orgaos and lowland Atlantic forest to the south, epilithic and epi¬phytic, at c. 100-1500 m altitude.

Desc from Hunt 2006
Branches up to 30cm or more, becoming pendulous, scarcely woody at base; branch segments oblong, truncate, 2.5-8 x 1-4 cm, margins sharply serrate-dentate with 2-4 forward pointing teeth each side, midrib prominent; areoles apical and marginal; flowers zygomorphic, 7 x 4.5cm, axis abruptly angled between pericarp and hypanthium; pericarp terete; hypanthium ca 4cm; perianth strongly oblique; tepals ca 3 x 1.5cm, recurved, variable in colour, stamens and style long exserted.

Seedlings of this species have the primary stem flattened from the beginning (observations are needed of seedling morphology in other species of the genus).