Shiki comes from the Japanese meaning to lay; when combined with futon, it is spelt and pronounced with a 'b', hence Shikibuton. It consists of one or two futon to form a mattress which is normally laid directly on the floor.

Japanese Traditional Futon, Soft Futon Floor Mattress, Breathable Tatami Fold Wool Wrap Mattress Thick Warm Bed Pad Double Sleeping Mat(Color:Grey,Size:Full)
3.0
Amazon.co.uk

Japanese Traditional Tatami Mat/Futon Mattress Firm And Comfortable Green/Natural Rush Grass/Folds Easily Great For Bed/Meditation Space/Yoga/Zen...
5.0
Amazon.co.uk

FULI Japanese Traditional Shiki Futon (shikibuton) High Grade Floor Mattress, 39" x 75" Single. Made in Japan
5.0
Amazon.co.uk