I thought is showed the ouroboros, a symbol of eternity, surrounding Isis. It appears I was wrong. A really good PhD thesis by Dana Reemes shows that the idea of the snake biting its tail as a symbol of eternity did not come in until much later. Instead, this snake, which isn't biting its tail is more likely to be a protective symbol surrounding the deceased as the solar-Osirian unity. Snakes like this appear on several coffins of the Priests of Montu at found at Deir el Bahri.
Moreover, this is actually the upper foot part of the coffin, designed so that it could be seen by the deceased. So, if we looked at it on the complete coffin it would appear upside down.
Anyway, if you are interested I have more on the fragment here.
