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"The X Digit"...a pteroid bone??

Hi folks,
I just got around to reading "The Beginnings of Birds" (Hect et. al.).
(Hey!...I told you I was playing "catchup" with my old paleontological
interests!).Anyways, ...got to the article by Hinchliffe 'One, Two, Three'
or 'Two,Three,Four', and it struck me immediately that my theory on "Birds
as Pterosaur Descendants" seemed to fit in...(like that annoying missing
piece of a large jigsaw puzzle that was never considered because of it`s
"unlikely" color).
The article describes how Holmgren (1955) and Montagna (1945) (quite a while
back)..."claimed to find either a transient digit or its associated distal
carpal preaxial to that developing digit which eventually forms the anterior
digit of the adult wing". This transient element, I call "X", because I
agree with the paleontological view that it cannot be called digit 1, as
digits 1,2 and 3 are already identified  (via phalangeal count), in both
Theropod manus and Avian wing. Yet, it has to be something, as embryologists
have determined its presence.
...There`s only one creature, (that I`m aware of), that fits the bill as
having and "extra" preaxial "digit", and that would be the Pterosaur, with
its "pteroid bone".