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... in arvis, et Mysum capitisque sui ripaeque prioris paenituisse...

'capitis': from 'caput' = head; I gather, this is another word for 'source of a river'; the same as 'os, -ris' used by Ovid a few vss above; the genitive here is governed by 'paenituisse' in the next verse: to have regretted its source and its former bank; I take it, that Ovid means here, that this river first ran through another place and later on changed its course.