Mammillaria longimamma v. uberiformis
(Dolicothele longimamma)

Skill level:
Germination: intermediate
Growing: intermediate
Use a free-draining mineral soil mix and plant in a deep pot to accomodate the large taproot of this Mexican species. Water during spring and summer but keep dry in winter. Fertilize every 2 weeks with quarter-strength high potassium fertilizer. Avoid exposing to full sunlight; this species needs light shade to avoid sun scalding. It will tolerate temperatures near freezing if soil is kept dry in winter.

Germination of cactus seeds is about timing, temperature, quality of seeds, cleanliness, and establishing a technique for sowing. Some species have a higher germination rate, and/or a lower germination time than others. For that reason a specific success rate can't be guaranteed. If you are having trouble getting any of your seeds to germinate, please let me know.

Growing from seed info: https://cactiguide.com/article/?article=article21.php

Some info courtesy of http://www.llifle.com