Arms of Geoffrey de Geneville (1225/33 - 1314) from the Dering Roll (c. 1270-1300)
Blazon: Azure three horse-barnacles expanded or, on a chief ermine a demi-lion rampant issuant from the partition line gules
There is an actual crustacean known as the horse barnacle (Semibalanus cariosus), but 1. the depiction here doesn’t look anything like an actual barnacle and 2. they are native to the northwestern Pacific, so absolutely nowhere near England or France, where they were mostly featured in heraldry. What this charge actually depicts is a slightly more complicated question. It was possibly originally a farrier’s tool to hold horses’ muzzles closed, but it does seem to have been used as a torture method in a few instances. In one of those cases, that of Sir Henry Wyatt, the family allegedly incorporated a horse-barnacle into their arms as a symbol of their loyalty to the Tudors.
The Genevilles, or Joinvilles as they were known in France, don’t seem to have a dramatic origin story for their arms, but there are plenty of other notables in the family to make up for any shortfall. Geoffrey’s older brother Jean de Joinville wrote a very well-known chronicle of the life of Louis IX and the Seventh Crusade, commissioned by Jeanne of Navarre. Geoffrey arrived in England along with Eleanor of Provence when she came to marry Henry III, and was later involved in a number of important negotiations, including with Llewelyn ap Gruffudd. He also traveled widely, going on diplomatic missions to Paris and participating in the Eighth Crusade. (No word on whether Jean warned him that was a terrible idea.) He had an exceptionally long life for the period, so it’s perhaps not surprising that his eldest son predeceased him. Upon his death, most of the Geneville titles and lands (plus the de Lacy properties Geoffrey inherited from his wife Maud) passed to Roger Mortimer via his wife Joan de Geneville, Geoffrey’s eldest granddaughter. She did eventually get her inheritance back after Roger’s execution for treason.