The family
The Montagus were a prominent medieval noble family particularly associated with the earldom of Salisbury. Their connection with the Monthermers arose in the fourteenth century through the marriage of John de Montagu to Margaret de Monthermer, through whom the Monthermer inheritance and arms passed into the Montagu family.
The Montagu earls became important figures in national affairs. Thomas Montagu, 4th Earl of Salisbury, was a distinguished soldier during the Hundred Years War and died in 1428. His only legitimate child, Alice Montagu, carried the Salisbury inheritance into the Neville family through her marriage to Richard Neville. Their son was Richard Neville, the celebrated “Kingmaker”.
The arms
The Montagu arms were Argent, three fusils conjoined in fess Gules — three red elongated diamond shapes joined horizontally on a silver field.
Monthermer bore Or, an eagle displayed Vert, beaked and membered Gules — a green eagle with wings spread on a gold field, with red beak and legs. The combined shield quarters Montagu with Monthermer.
Salisbury and Wiltshire
The association with Salisbury is unusually direct: the Montagus themselves held the earldom of Salisbury, and John de Montagu was buried in the Lady Chapel of Salisbury Cathedral.
On the St Thomas’s ceiling
The Montagu and Monthermer arms appear at:
Both RCHME and Buckeridge identify the shield as Montagu and Monthermer.
Sources
RCHME and Buckeridge — identification of the St Thomas’s ceiling heraldry.