Thomas, a student from Llanelli who is only on a summer contract with Glamorgan, claimed 5 for 80 from 29.2 overs. Those were also the best figures on a Championship debut for the county since Ezra Moseley took 6 for 102 against Essex at Swansea in 1982.
Nevertheless, Derbyshire established a first-innings lead of 164, having looked in difficulties when, with Chris Adams absent ill, they stood at 167 for 5, three runs behind. At that stage, Derbyshire were still three runs behind but Frank Griffith and Ian Bishop siezed the initiative with a stand of 142 in 49 overs, Griffith hitting a career-best 81 and Bishop 90 before the home side declared at 334 for 8. Glamorgan were 46 for 1 at the close.
Hampshire's all-rounder Jon Ayling took a career-best 5 for 12 at Bournemouth as Middlesex were bowled out for 115 and had to follow on 271 runs behind. Middlesex did better the second time around, reaching 153 for 2 by the close.