Credner, basing his views on the discovery by him of various annectent forms between the Stegocephalia and the Rhynchocephalian reptiles, has proposed a class, Eotetrapoda, to include these forms, ancestors of the batrachians proper on the one hand, of the reptiles proper on the other.
Yet, that the Stegocephalia, notwithstanding their great affinity to the reptiles, ought to be included in the batrachians as commonly understood, seems sufficiently obvious from the mere fact of their passing through a branchiate condition, i.e.
The outcome of our present knowledge points to the Stegocephalia, probably themselves derived from the Crossopterygian fishes (8), having yielded on the one hand the true batrachians (retrogressive series), with which they are to a certain extent connected through the Caudata and the Apoda, on the other hand the reptiles (progressive series), through the Rhynchocephalians and the Anomodonts, the latter being believed, on very suggestive evidence, to lead to the mammals (9).
In the earliest forms of this order, the Stegocephalia, we meet with considerable variety in the constitution of the vertebrae, and these modifications have been used for their classification.
Although not strictly forming part of the skull, allusion should be made here to the ring of sclerotic plates which has been found in many of the Stegocephalia, and which is only found elsewhere in a few Crossopterygian fishes as well as in many reptiles and birds.