High Sheriff of Westmorland

http://en.wikipedia.org/wiki/High_Sheriff_of_Westmorland
before 1283 Thomas de Mussegrave D/WYB/2/38

According to this Wiki article, Thomas de Musgrave was High Sheriff of Westmorland until 1283 and Richard (d.1301) does not appear on the list. The Wiki author states records are from Public Records Office in Kew and admits his list is incomplete.

Here are my quesitons.
1. WHICH Thomas was Sheriff before 1283? "Before" 1283 -- so when was he Sheriff?
Sir Thomas de Musgrave, Knight, Sheriff of Westmorland
Sir Thomas de Musgrave, of Morton, Knight

2. Has Richard been left off the list or was he never Sheriff.
Richard de Musgrave, Sheriff of Westmorland