The "True" Christmas Cactus
Schlumbergera x buckleyi

Schlumbergera truncata was originally named Cactus truncatus. It later joined Epiphyllum with E. Russellianum. Both, along with their hybrids, were subsequently transferred to Zygocactus, and then to Schlumbergera.

The Florist and Pomologist 1: 13-15 (Jan 1868)
Epiphyllum truncatum and its varieties
William Buckley
Garden Companion and Florists' Guide p. 45 (Jan to Oct 1852)
3. Epiphyllum Buckleyi 2. E. Rollissonii
(both from Russellianum x Truncatum)
THE Epiphyllum truncatum has always, since its first introduction from Brazil in 1818, received the attention of cultivators, and many a hoary-headed son of Adam has pointed with pride to the big Cactus at the end of the vinery, now all a-blaze with blossom, that had been grafted by himself some twenty years before. At intervals a few others were obtained, as E. truncatum bicolor, Bridgesii, and rubro-tinctum; and in 1839 the lovely E. Russellianum made its appearance, also from Brazil. This latter, although recorded as a variety of E. truncatum, is certainly a distinct species, for while the varieties of E. truncatum usually flower in November and December, the natural blooming period for E. Russellianum is the month of May. There are, besides, other differences, the petals being evenly reflexed, the stamens straight, and the seed-vessel angular; while in E. truncatum the flower is ringent, the stamens curved, and the seed-vessel smooth. Moreover, the thick leafy branches are less toothed than those of E. truncatum, and only one-third of their size.

The late Mr. Hamp, of Mawbey House, Stockwell, tried hard to obtain a hybrid between Epiphyllum truncatum and Cereus speciosissimus, but could never succeed. He did, however, raise one good variety of truncatum, named magnificum. Mr. Bruce, also, the talented gardener at Collier's Wood, Merton, tried in vain to produce a hybrid between the E. truncatum section and E. speciosum and others, although he obtained some beautiful hybrids in other sections. It may, therefore, be concluded that E. truncatum will not hybridise with the large-flowered species. More recently, some very beautiful hybrids were raised at the Tooting nursery between the E. Russellianum and E. truncatum, having the symmetrical form of the first, and flowering two months later than the last. The advantage gained by this cross was important, inasmuch as it extended the blooming time quite through the winter, to say nothing of the superiority of form which was secured.

Curtis's Botanical Magazine 66: 3717 (1839)
Epiphyllum russellianum. The Duke of Bedford's Epiphyllum
Curtis's Botanical Magazine 53: t. 2562 (1825)
Cactus Truncatus. Ringent-Flowered Cactus