* David Bull kindly emailed me to let me know that the sack I refered to is actually (kagami mochi) two rice cakes stacked up "... a traditional new year decoration. The lines are the traditional way of drawing cracks in the surface of the cake ... as it dries out (it isn't eaten) the outer surface becomes hard and cracked ..." Here's a link to The Japan Exchange and Teaching Programme with more info. Thanks Dave!
I should also have said that my print was based on 2 designs by Woodblock Artist Kawanabe Kyosai (1831-1889) which I combined in Photoshop before printing it out and pasting it onto the blocks for carving.
Kyosai's work is full of weird ghosts and creatures, and his anthropomorphic animals are great fun. He's little known in the West compared to Hokusai and others, but shouldn't be overlooked as a great source of inspiration.