  We show that smooth curves in the same biliaison class on a hypersurface in
$\mathbf{P}^3$ with ordinary singularities are linearly equivalent. We compute
the invariants $h^0(\mathscr{I}_C(d))$, $h^1(\mathscr{I}_C(d))$ and
$h^1(\mathscr{O}_C(d))$ of a curve $C$ on such a surface $X$ in terms of the
cohomologies of divisors on the normalization of $X$. We then study general
projections in $\mathbf{P}^3$ of curves lying on the rational normal scroll
$S(a,b)\subset\mathbf{P}^{a+b+1}$. If we vary the curves in a linear system on
$S(a,b)$ as well as the projections, we obtain a family of curves in
$\mathbf{P}^3$. We compute the dimension of the space of deformations of these
curves in $\mathbf{P}^3$ as well as the dimension of the family. We show that
the difference is a linear function in $a$ and $b$ which does not depend on the
linear system. Finally, we classify maximal rank curves on ruled cubic surfaces
in $\mathbf{P}^3$. We prove that the general projections of all but finitely
many classes of projectively normal curves on $S(1,2)\subset\mathbf{P}^4$ fail
to have maximal rank in $\mathbf{P}^3$. These give infinitely many classes of
counter-examples to a question of Hartshorne.
