Spectator 2214

[Matakari]

Seeking some hints for the following, please:

ACROSS:
12 I forgot to say, mouths itch(5): ?S?R?
15 Antelope, or a dozen before noon(6): ??E?E?
30 Run round in sierra, confusing muleteers(8): A??????S
37 Bone's massive, so on large creatures back(7): O??????
39 Where number is claimed to change outside of polygonal one( 12, 2 words):?????A?????E

DOWN:
11 County runners taking pressure for us(9): C??P?????

Many thanks in advance!

[kayakamina]

12a psora
15a dzeren
30a arrieros
37a 0ssicle
39a decimal place
11d compilers

[Neveracrossword]

Definitely hints?

30 An anagram of 'sierra' and the usual crossword depiction of 'round'.

12. The usual two-letter abbreviation for 'I forgot to say', added to a letter, perhaps+ a 3-letter word for 'mouths'.

[Matakari]

Thank you, Kayakamina and Neveracrossword!