The right and left halves are completely divided by septa, no mixture of the venous and arterial blood being possible, an advance upon reptilian conditions, even the highest.
Archaeopteryx was a bird, without any doubt, but still with so many low, essentially reptilian characters that it forms a link between these two classes.
More dimly still visions of what the first bird may have been like could be reasonably entertained; and, passing even to a higher antiquity, the reptilian parent whence all birds have sprung was brought within reach of man's consciousness.
The lizards and snakes are the two dominant reptilian orders which are still on the increase in species, though certainly not in size.
An osteological question which has been much discussed is the fate of the reptilian quadrate bone in the mammalian skull.
Others however (inclusive of Tritylodon and Microlestes, if they be really mammals), seem nearer to the Monotremata; and the question has yet to be decided whether placentals and marsupials on the one hand, and monotremes on the other are not independently derived from reptilian ancestors.
The gradual passage of the thick root of the tail into the body is a character common to the Tasmanian wolf and the aard-vark, and may be directly inherited from reptilian ancestors (see Thylacine).
Osborn, the tripartite reptilian condyle, by the loss of its median element, has given rise to the paired mammalian condyles; so that this difficulty disappears.
The muscles of the limbs show a great amount of specialization, away from the fundamental reptilian and mammalian conditions.
At any rate, there seems little doubt that it was the region where creodonts and other primitive mammals were first differentiated from their reptilian ancestors.