The Collaborative International Dictionary of English v.0.48:

Chelonian \Che*lo"ni*an\, a. (Zool.)
   Of or pertaining to animals of the tortoise kind. -- n. One
   of the Chelonia.
   [1913 Webster]

WordNet (r) 3.0 (2006):

chelonian
    adj 1: of or relating to or resembling or being a turtle or
           tortoise
    n 1: a reptile of the order Chelonia [syn: chelonian,
         chelonian reptile]