  We present a direct and fairly simple proof of the following incidence bound:
Let $P$ be a set of $m$ points and $L$ a set of $n$ lines in ${\mathbb R}^d$,
for $d\ge 3$, which lie in a common algebraic two-dimensional surface of degree
$D$ that does not contain any 2-flat, so that no 2-flat contains more than $s
\le D$ lines of $L$. Then the number of incidences between $P$ and $L$ is $$
I(P,L)=O\left(m^{1/2}n^{1/2}D^{1/2} + m^{2/3}\min\{n,D^{2}\}^{1/3}s^{1/3} + m +
n\right). $$ When $d=3$, this improves the bound of Guth and Katz~\cite{GK2}
for this special case, when $D$ is not too large.
  A supplementary feature of this work is a review, with detailed proofs, of
several basic (and folklore) properties of ruled surfaces in three dimensions.
