An instantly recognizable type of small female stone figure characterized by a short rounded body, a wide-eyed expression, and a giant vulva held open with both hands. These are found all over England and Ireland; the name is thought to come from Gaelic. One might not expect to find them adorning Christian churches, but they do show up (especially in Norman-built churches) though many have been removed -- they are no longer as common as the less conspicuous Green Man.
It is unknown whether the sheela-na-gig represents a goddess or class of goddesses, or the general idea of fertility or sex or good luck (not as simple as one might expect, since many of them are made to look very old and slightly hostile), or something else entirely. If she were part of a pantheon or a zodiac, one might expect to find other recognizable characters in the same style but there are none. Enthusiastic neo-pagans have taken this as evidence that primitive Celts and Europeans worshiped a single mother goddess.
There is an obvious reference to the squat shape of sheela-na-gigs in Chapter 15 when Riddley refers to "dollys" and draws their outline. Riddley never mentions a connection between these fertility figures and the sexually voracious death-hag Aunty, but it is possible to imagine them devouring as well as nurturing. EB
(Note, however: "An Anglo-Saxonist archaeologist friend of mine has done a lot of research on the sheela-na-gigs and they aren't all 'dolly-shaped'; in fact there's one on the door of one of the [men's, oddly enough] lavatories at University College, University of Toronto." SLK)