The affix ḍaṭ (a causing the elision of the last vowel with the consonant following it) comes in the sense of 'making full this number' after a Numeral being the Genitive 6th-Case in construction.,
That by which a thing is completed is called पूरण ॥ Thus एकादशानां पूरणः = एकादशः the \eleventh\ (i. e. the one which, added to ten, completes the eleven). Similarly त्रयोदशः ॥ By this affix, ordinals are formed. The rule therefore, does not apply to a case like this:- पंचानां मुष्टिकानां पूरणो घटः ॥,