Letters, notes, articles, drafts and other papers, 1926-1939, relating to the work later published as Animal Call-Words: a study in human migration, published in 1939; folklore material collected by some Cardiganshire schools for the Cardiganshire Schools Folklore Collection and school inspectors' reports, 1917-1926; papers relating to David Thomas's interest in the philological history of Cardiganshire, with particular reference to the Goidelic language, 1924-1936; circulars, reports and correspondence of the Cardiganshire War Savings Committee, 1916-1918; papers of the Cardiganshire Antiquarian Society, 1910-1939; the National Library of Wales Building Fund, 1928-1930; personal papers comprising papers from his professional and academic career, 1885-1926, estate papers of his brother Daniel Jones Thomas (d. 1926), and family papers, [c. 1886]-1928; and miscellanea including manuscripts of David Thomas of Nantygwydde, 1830-1847, Mary Thomas of Pantyclochydd, 1926, Dr G. Arbour Stephens, 1912-13, J. Toriel Williams, [c. 1942-3], and J. W. Jones of Blaenau Ffestiniog, 1881-1934, together with press cuttings, letters to D.T., and various notes, pamphlets and verses.