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Doesn't the angle at the base of the pinnules, which is far more than 90 degrees, rule out aculeatum?
... don't the singly pinnate fronds rule out setiferum?
(I would argue that the angle at the base of the pinnules here is not homologous to the angle usually used in distinguishing aculeatum and setiferum, and that it is necessary to be cautious in using it.)
... Polystichum setiferum is only known from a single site in Skye (near Portree).
This clearly demonstrates the problems in trying to ID immature ferns!
but having looked round images on the web, it doesn't look like any of the hybrids, and there are some similar plants labelled as aculeatum. On grounds of morphology and distribution I think that on balance it is aculeatum.
I agree that this is a young P aculeatum. We often go out monitoring P lonchitis on the limestone pavements, and the most usual cause of confusion is with young P aculeatum
Lat/Lng: 57.235, -5.917
OS grid ref: NG637228