<a target='_blank' href="
http://www.amazon.com/gp/product/0879518391/10 2-1641069-2862553?v=glance&n=283155">Th e Flight of Dragons</a>, a wonderfully illustrated book on dragon theory, states that a reptile (a vertebrate) with 6 limbs (4 legs and 2 wings) is not biologically consistent. (A platypus may be able to lay an egg, but there are no 6-legged reptiles that i know of.)
A dragon with 4 limbs (2 legs and 2 wings with bat-like claws) would be more in line with the rest of earth's creation.
Of course, that doesn't stop the 4-legs-2-wings tradition. Anne McCaffrey, the 'Queen of Dragons', describes 6-limbed dragons in her books.