We were somewhere around Barstow, on the edge of the desert, when the
drugs began to take hold. I remember The Natural Philosopher
saying something like:

The general guide - not rule - is to follow the spoken form. If you
would say 'denisses schizophrenia' then dennis's is the most apprropiate
form, but of you were to say 'dumars books' you would make it Dumas' books.

Since no one hear ever talks about dennis, its hard to say which would
work better.

"Dennis's arse in parsley" sounds about right.