Arms: Quarterly: 1 and 4, Gules, a lion rampant Or (FitzAlan); 2 and 3, Sable, fretty Or (Maltravers).
Plain English: A quartered shield combining a gold lion on red for FitzAlan with a gold interlaced lattice on black for Maltravers.
The family
The FitzAlans were one of the great noble families of medieval England and held the earldom of Arundel. Their connection with the Maltravers family arose through the marriage of Sir John Arundel to Eleanor Maltravers, heiress of the Maltravers estates, in the late fourteenth century. Their descendants inherited both the Maltravers title and, eventually, the earldom of Arundel. Wikisource
During the period of the St Thomas’s ceiling, William FitzAlan (1417–1487) was Earl of Arundel and Baron Maltravers. He was succeeded by his son Thomas, who lived until 1524.
The arms
The FitzAlan coat was Gules, a lion rampant Or — a gold lion standing upright on a red field.
The Maltravers arms were Sable, fretty Or — a black field covered by an interlaced gold lattice. Medieval representations sometimes reduce the lattice to a single gold fret. The FitzAlans quartered the two coats, placing FitzAlan in the first and fourth quarters and Maltravers in the second and third. This combination is recorded on the seal of John FitzAlan, Earl of Arundel, about 1432. Wikisource
Salisbury and Wiltshire
Unlike several families represented elsewhere on the ceiling, no strong direct connection between the FitzAlan Earls of Arundel and fifteenth-century Salisbury has yet been established.
On the St Thomas’s ceiling
The FitzAlan and Maltravers arms appear at:
Both RCHME and Buckeridge identify the shield as FitzAlan and Maltravers, making this a well-supported heraldic identification despite the reason for its presence at St Thomas’s remaining uncertain.
Sources
RCHME and Buckeridge — identification of the St Thomas’s ceiling heraldry.
Arthur Charles Fox-Davies, A Complete Guide to Heraldry — FitzAlan and Maltravers arms.