Annona squamosa × A. cherimola "Pinks Mammoth" Atemoya Seeds
Atemoya cultivars are not known to grow true to seed, you must propagate this cultivar vegetatively (grafting, air layer, etc.) to call your plant a true Pinks Mammoth rather than "a seedling of" Pinks Mammoth. That said, the seedlings will likely be of high quality.
Information from Purdue University:
An early hybrid that arose in Queensland after the introduction of cherimoya seeds from South America, was named 'Mammoth' (or 'Pink's Prolific', or 'Pink's Mammoth') and became the basis of the commercial production of atemoyas there and on the north coast of New South Wales, though the flesh of this cultivar immediately below the rind is usually brownish and bitter.
The tree closely resembles that of the cherimoya; is fast-growing; may reach 25 to 30 ft (7.5-9 m) and is short-bunked, the branches typically drooping and the lowest touching the ground. The leaves are deciduous, alternate, elliptical, leathery, less hairy than those of the cherimoya; and up to 6 in (15 cm) in length. The flowers are long-stalked, triangular, yellow, 2 3/8 in (6 cm) long and 1 1/2 to 2 in (4-5 cm) wide. The fruit is conical or heart-shaped, generally to 4 in (10 cm) long and to 3 3/4 in(9.5 cm) wide; some weighing as much as 5 lbs(2.25 kg); pale bluish-green or pea-green, and slightly yellowish between the areoles. The rind, 1/8 in (3 mm) thick, is composed of fused areoles more prominent and angular than those of the sugar apple, with tips that are rounded or slightly upturned; firm, pliable, and indehiscent. The fragrant flesh is snowy-white, of fine texture, almost solid, not conspicuously divided into segments, with fewer seeds than the sugar apple; sweet and subacid at the same time and resembling the cherimoya in flavor. The seeds are cylindrical, 3/4 in (2 cm) long and 5/16 in (8 mm) wide; so dark a brown as to appear black; hard and smooth.
Origin and Distribution
The first cross was made by the horticulturist, P.J. Wester, at the United States Department of Agriculture's subtropical laboratory, Miami, in 1908. Seedlings were planted out in 1910. Other crosses made in 1910 fruited in 1911 and seeds were taken by Wester to the Philippines. The hybrids grew there to 7 1/2 ft (2.3 m) high in one year, had to be moved to another location; one bloomed in 1913 and was pollinated by the custard apple, q.v. The rest of the plants fruited in 1914. Resulting fruits were superior in quality to the sugar apple and were given the name "atemoya", a combination of "ate", an old Mexican name for sugar apple, and "moya" from cherimoya. Cuttings of 9 of the hybrids were sent by Wester to the United States Department of Agriculture in January of 1915. (S.P.I. Nos. 39808-39816), #39809 representing the hybrid tree pollinated by the custard apple. In 1917, Wester sent cuttings of #39809 under the name "cuatemoya" to the United States Department of Agriculture (S.P.I. Nos. 44671-44673). In the meantime, Edward Simmons, at the Plant Introduction Field Station, Miami, had successfully grown hybrids and they had survived an early February 1917 drop in temperature to 26.5ºF (-3.10ºC), showing the hardiness derived from the cherimoya. Another introduction was received from the Philippines in 1918 (S.P.I. #45571). A few experimental growers in southern Florida maintained atemoya trees (apparently distributed by the United States Department of Agriculture) for many years while there was a general lapse of interest in this fruit. Today, there are a few small commercial plantings and the fruits are being sent to some northern fruit dealers. In the early 1930's or 1940's, what were apparently chance hybrids between adjacent sugar apple and cherimoya orchards attracted attention in Israel and work was begun to choose and standardize the best of these for vegetative propagation.