 Corythosaurus started developing its crest when half the size of adults , but Parasaurolophus juveniles grew crests when only 25 % as long as adults . <unk> Corythosaurus , along with adults , had a premaxilla @-@ nasal <unk> . Young and adult Corythosaurus are similar to Lambeosaurus and Hypacrosaurus but <unk> to Parasaurolophus in that the <unk> of the skull are <unk> , and not smooth and straight . This feature helps differentiate <unk> from <unk> . Generally , the crests of juveniles of lambeosaurines like Corythosaurus , Lambeosaurus , Hypacrosaurus <unk> , <unk> like Parasaurolophus , and primitive lambeosaurines like <unk> are quite alike , although other features can be used to distinguish them . 
