Both forms are acceptable. Traditionally, the former has been used in the past, while the latter form is now more grammatically correct, according to most sources.
Edmund Bartley-Denniss died in 1931.
Edmund Bartley-Denniss was born in 1854.
Gary Denniss has written: 'Macaulay Township in days gone by'
Denniss Wade was born on December 31, 1979, in Los Angeles, California, USA.
James Denniss ate it
Please send the report to Dennis and I by Friday.
his name is Dennis Mitchell
Dennis'
Dennis is the scion of the Mitchell family.
Dennis's last name is "Mitchell".
Dennis = Kenika
You use Dennis's when meaning, "belonging to Dennis." It's the possessive form of Dennis, which is a name.