You may already have the answer to your question since it was posted a while back, but I was very excited to find out that we are cousins!
Your message:My mother's grandmother was Katherine Agnes Whitman. Her Father was Martin Ira Whitman (b. 7 APR 1843 in South bend Indiana) who married Lydia Alice Barnum Clark. Martin's father was William Green Whitman of Vermont who married Laura Jane Finch. That's as far back as I can go so far.
My father's grandmother was also Katherine Agnes Whitman whose husband was Lucius Cary Slayton and son was Harry Whitman Slayton whose son was William Cary Slayton who was my father.
I have information that I found in a reference book a few years ago called, "First Families of America" and it has listings of a lot of DAR members, one of which was Katherine.It lists the Whitman line all the way back to Ens. John Whitman.There is NOT a lot of information about each of them other than some information on marriages and military experience.It also traces the line of her mother, Lydia Alice Cecilia Clark, daughter of Aaron B Clark and Mary Ann Randolph.
I would be glad to email you the page at work next week if you are still interested.
Also, do you have more information on Katherine & her parents?