  The purpose of this article is to investigate the combinatorial properties of
the cross section lattice of a $J$-irreducible monoid associated with a
semisimple algebraic group of one of the types $A_n$, $B_n$, or $C_n$. Our main
tool is a theorem of Putcha and Renner which identifies the cross section
lattice in the Boolean lattice of subsets of the nodes of a Dynkin diagram. We
determine the join irreducibles of the cross section lattice. Exploiting this
we find characterizations of the relatively complemented intervals. By a result
of Putcha, this determines the M\"{o}bius function for $\Lambda$. We show that
an interval of the cross section lattice is Boolean if and only if it is
relatively complemented if and only if it is atomic. We characterize
distributive cross section lattices, showing that they are products of chains.
We determine which cross section lattices are supersolvable, and furthermore,
we compute the characteristic polynomials of these supersolvable cross section
lattices. At the end of the article we indicate some future research
directions.
