Θα συμφωνήσω κι εγώ με το -η-, και όχι μόνο λόγω Πτολεμαίου.
Το OED λημματογραφεί στο Demetian, παρότι παραδείγματα με -e- είναι μόνο τα πιο πρόσφατα.
Demetian, a. and n. (dɪˈmiːʃɪən)
Also Dimetian.
[f. Demetia, Dimetia, Dyfed in S.W. Wales (Cardigan, Pembroke, and Carmarthen) + -ian.]
Of or pertaining to Demetia in South Wales; also as n., the dialect of this region.
1841 Anc. Laws & Inst. Wales Pref. xii, The Dimetian or West Wales Code. 1877 H. Hicks in Q. Jrnl. Geol. Soc. XXXIII. 230, I propose now to divide the Pre-Cambrian rocks into two distinct series under the local names of Dimetian (Dimetia being the ancient name for a kingdom which included this part of Wales) for the lower, and Pebidian‥for the upper series. Ibid. 231 The rocks which compose the Dimetian series are chiefly compact quartz schists. Ibid. 238 My endeavour has chiefly been to point out spots where the unconformity between them [sc. Pebidian rocks] and the Cambrians above, or the Dimetians below, is well marked. 1885 A. Geikie Text-bk. Geol. (ed. 2) 639 The so-called ‘Dimetian’ I regard as a granite which has invaded the Cambrian rocks. 1910 Encycl. Brit. XIII. 838/2 Three [legal] codes‥called Venedotian, Demetian and Gwentian, are said to have been written down by Bleggwryd, archdeacon of Llandaff. 1913 J. M. Jones Welsh Gram. 8 Demetian, the dialect of Dyfed or South West Wales. Ibid., Dialectal forms, chiefly Demetian and Powysian -e, begin to appear in the MSS. of the 15th century.
