The first symbol of St Hilda's Hall was the ammonite, a fossil consisting of whorled chambered shells, once supposed to be coiled snakes petrified. The early seventeenth century Lives of Women Saints of our Contrie of England explains their association with St Hilda: 'In that monasterie of Whitbye, there were such aboundance of serpents, what throughe the thicknes of bushes, and the wildernesse of the woods, that the virgins durst not peepe out of their Cells, or goe to draw water: but by her prayers she obtayned of god, that they might be tourned into stones; yet so as the shape of serpents still remayned; which to this day, the stones of that place do declare, as eye-witnesses haue testified.'
In The Cheltenham Ladies' College Magazine in 1893, Dorothea Beale wrote of the new St Hilda's in Oxford: 'We think of adding to our emblem, which must of course be an ammonite, the motto "non frustra vixi," – the old legend scarcely commends itself to modern ideas, but a fossil suggests the thought that every living thing, in passing away, should leave some work finished, something which remains for beauty, or for use.' The use of the ammonite with the motto 'I lived not in vain' has continued in use, with varying degrees of popularity, throughout St. Hilda's history.
When the College was incorporated in 1926 it could not afford a coat of arms, but a common seal was designed by Edmund New, with a bookplate, note paper and blazer badge based upon it. This remained the usual emblem of the College until the coat of arms was granted in 1960. The motto was not included in the grant of arms, although it is occasionally used.