Teenage all-rounder Kiran Carlson signs a new three-year contract with Glamorgan.
Glamorgan batsman Will Bragg announces his retirement because of illness after 11 seasons of first-class cricket.
Glamorgan begin their 2018 Championship campaign against Gloucestershire at Bristol, followed by facing Middlesex at Lord's.
County champions Essex begin the defence of their title in 2018 against Yorkshire at Headingley on 13 April.
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Glamorgan coach Robert Croft says he is thrilled to sign Australian batsman Shaun Marsh as Glamorgan's new overseas player.
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How to get into cricket - physical exertion with a chess-like element in matches which range from half an hour to five days.