Indeed we need to remember a lot as evolved from the early drafts to Alien.... we had Urns not Eggs.... they was leathery but the other was described as hard.
As SM points out however... it is referenced that they are the same, and so these leathery urns once they have done their job they could dry out and go hard.... this would explain the difference.
We have to remember the final designs of the Eggs was not know to the author and so they was just describing things related to the earlier drafts.
The movie with Gigers Help gave us the Eggs.
Prometheus used the Urns descriptions to be a basis for its Bio-Weapons... as movies in the Franchise have re-used concepts and designs from previous concept and draft work.
Star Beast Mural Chest Buster is visually a lot like the Adult Trillobite.
I cant wait until Ridley finishes his Franchise and we get more answers that would clear things up.... it would stop difference of opinions which can be a good or bad thing too.
We have to remember however that the Movie is Canon and its what the movie showed that we need to consider and not the Novelization.
However yes they could run with the Urn in the Main Room which evolved to the Pilot Room for Alien....
LV-223 is implied in a connection to LV-426 and so yes a Urn could have started the whole thing.... so its possible something related to the Urns infected the Space Jockey that led to a Chest Buster that melted its way down to the Cargo Hold and either Laid Eggs or Evolved the Urns to Eggs.
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