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A Clutter of Duckbills

Referring to two lumps of hadrosaurian duckbills.

I have looked at photos of fossils, skeletal drawings, and fine
reconstructions of several duckbilled dinosaurs.  Two groups of closely
related genera and species.  I have thought about it, and it seems odd that
such closely related animals are kept apart.  

Hypacrosaurus, Corythosaurus, and Lambeosaurus, for example.  Without the
crests, it is rather hard distinguishing one animal from the other.  Indeed,
one juvenile AMNH specimen of a lambeosaurine duckbill could easily have
either been a Corythosaurus or a Lambeosaurus (I think the final arguments
placed it in Lambeosaurus).  Thats how undistinguishable those species are.
 What details keep this lump of animals in different genera?  Actually, what
kind of details keep one species in one genera or the other?  The details on
these three lambeosaurine genera certainly indicate how closely related they
are.  Hypacrosaurus and Corythosaurus have the same type of crest, the
"helmet" crest, and Lambeosaurus has a similar crest except that the
lambeosaurus crest has a thin spine sticking backwards on it.  Without these
crests, the rest of the skeleton indicate that these animals had probably
evolved from a recent common ancestor a few million years before they came

What keeps Lambeosaurus, Corythosaurus, and Hypacrosaurus in different
genera?  Stanley Friesen suggested to me that all of the species of these
three genera may actually belong to only one genus -- the first one coined,

Sounds like a decent lumping to me.  (Much more probable that lumping
Deinonychus into Velociraptor.)

Another group of hadrosaurs that confuse me are Gryposaurus, Hadrosaurus,
Kritosaurus, and Brachylophosaurus.  I know little about these genera in
particular, except for the fact that they are so closely related and similar.
 Any thoughts about which genus and species should go where?  Which one was
coined first?  Hadrosaurus was the first good hadrosaur skeleton to be
discovered in N. America.  Just how good was it?  Was it good enough to show
that Gryposaurus, Kritosaurus, and Brachylophosaurus were actually different
species of Hadrosaurus?  

Just what keeps two species in the same genus?  And just what keeps them

Raptor RKC (Rachel Clark)