Cassidy, the first "ghost" hired to work in the Siegel and Shuster studio, died May 15.
He's the artist credited with putting an "S" insignia on Superman's cape.
From a story in the Milwaukee Sentinel:
Cassidy arrived in Milwaukee as a graphics arts teacher at the old Milwaukee Vocational School, a forerunner of Milwaukee Area Technical College. Superman's creators - writer Jerry Siegel and artist Joe Shuster - spent part of the 1930s promoting Superman as a comic strip character. They finally began to find some success and moved into comic books in the late 1930s.
...Thanks to a college contact, Cassidy was offered the chance to join the Shuster studio in Cleveland when help was needed on the comic books. He worked with Shuster from 1938 to 1940. The money wasn't nearly enough to support his young family, so Cassidy had to stop working full time at the studio.
"It paid so little, he went into teaching," said Dick Cassidy, repeating the family joke.
Cassidy went on to work with Field Enterprises in Chicago, as an artist for the World Book Encyclopedia, and as art director and managing director for its Child Craft publication. He later worked for Grolier's Book of Knowledge in New York City but didn't like the job or living in New York.
There's more info on Cassidy at the excellent Who Drew Superman site.