Diph*thon"gal*ly

from The Collaborative International Dictionary of English v.0.48

Diphthongal \Diph*thon"gal\ (?; 115), a.
   Relating or belonging to a diphthong; having the nature of a
   diphthong. -- {Diph*thon"gal*ly}, adv.
   [1913 Webster]