Dear Newsgroup ~
Complete Peerage 9 (1936): 140–145 (sub Monthermer) has a good account of Sir Ralph de Monthermer, 1st Lord Monthermer, sometime Earl of Gloucester and Hertford, who died in 1325. Sir Ralph de Monthermer's 1st wife was Joan of England, daughter of King Edward I of England, by which marriage he had two sons, Thomas, Knt., and Edward, Knt., and one daughter, Mary (wife of Duncan of Fife, Knt., 10th Earl of Fife). Further particulars of this family can be found in my book, Royal Ancestry (5 volume set), published in 2013.
In Complete Peerage 9 (1936): 143, footnote a, there is a long discussion as to whether Thomas or Edward was Sir Ralph de Monthermer and Joan's eldest son. This discussion reads as follows:
"It is not certainly known which of the sons Thomas and Edward was the elder. In the remainder to the grants to Ralph in 1309 and 1310 Thomas is named first, but he is not invariably given precedence. Thomas also appears to have been summoned for service at an earlier date than his brother; on the other hand, Edward was summoned to Parliament and Thomas was not, a circumstance which, however, has not the significance which it would have at a later period. It might be argued that Edward was the elder because he was apparently named after his royal grandfather; but if Ralph's father was named Thomas, it is equally probable that the first born was named after him." END OF QUOTE.
So back in 1936, it was left hanging as to whether Thomas or Edward was the eldest son of Sir Ralph de Monthermer, by Joan of England. Even so, the author in his main text "presumed" Thomas was the elder son and "presumed" Edward was the younger son. Sadly, no additional information on this matter was provided in the Addenda and Corrigenda volume of Complete Peerage [Volume 14] published in 1998.
So which son was the eldest? Thomas or Edward?
Recently I located a Common Pleas lawsuit which establishes once and for all that Sir Thomas de Monthermer was in fact the eldest son and heir of his father, Sir Ralph de Monthermer. A brief abstract of the lawsuit is provided below:
In Michaelmas term 1334 Richard de Thorp and Alice his wife sued Thomas son and heir of Ralph de Monthermer in the Court of Common Pleas in a Yorkshire plea regarding a debt of £90.
Reference: Court of Common Pleas, CP40/300, image 93 (available at
http://aalt.law.uh.edu/AALT1/E3/CP40no300/bCP40no300dorses/IMG_0093.htm).
We see here that Thomas de Monthermer is specifically styled "son and heir of Ralph de Monthermer." In this time period, the terminology "son and heir" is almost exclusively reserved for the eldest surviving son of a deceased parent.
Elsewhere, I find that in the following year, Hingeston-Randolph, Register of John de Grandisson Bishop of Exeter 2 (1897): 786 records "Thomas and Edward de Monthermer, Knights" (named in that order) presented to the church of Stokenham, Devon in 1335.
The above record may be viewed at the following weblink:
https://babel.hathitrust.org/cgi/pt?id=mdp.39015022703857&view=1up&seq=192
In summary, reviewing the evidence, I find it conclusive that Sir Thomas de Monthermer was the eldest son and heir of Sir Ralph de Monthermer, by his wife, Joan of England. He is so styled in a contemporary lawsuit dated 1334; he is named first ahead of his brother, Edward, in grants dated 1309 and 1310; he was summoned for military service first before his brother, Edward; and he is named ahead of Edward in the presentation to a church in 1335.
Best always, Douglas Richardson, Salt Lake City, Utah