Dürer's shell curve may be defined as follows: given a parabola and a line that is tangent to the parabola, it is the glissette of a point
on a line sliding between the parabola and the tangent. It has the equation

(*x*^{2} + *xy* + *ax* - *b*^{2})
= (*b*^{2} - *x*^{2})(*x* - *y* + *a*)^{2}.