?28 
MR. T. W. BRIDGE ON THE OSTEOLOGY OF POLYODON FOLIUM. 
admitting their community of descent. We must therefore conclude that the impor- 
tant differences between the two types are probably due to their early separation 
from each other, and to the fact that the lowest Urodela have undergone much retro- 
grade metamorphosis in their cranial structure, while adaptive changes have profoundly 
modified the adult and larval forms of both groups since they first came into existence. 
An alternative suggestion which may be made is to regard the Ganoid-Amphibian 
phylum as splitting into its two main constituent groups, the primeval Amphibia 
afterwards differentiating into Urodela and Anura, while the primitive root-stock of 
the Ganoids gave rise to the Teleosteoidei and to the Teleostei proper, the Selachoidei 
types persisting with relatively little modification as representatives of those early 
progenitors of their order. 
We may venture more clearly to express the foregoing conclusions as to the 
relations of Polyodon and the other Ganoids to the Amphibia and Elasmobranchii 
by the aid of the following diagram : — 
Elasmobranchii . 
Apncumatocoela — 
G. Selachoidei. G. Teleosteoidei. Urodela. Anura. 
Dipnoi (?) 
— Pneumatocoela. 
x. 
As to the nature of the primitive ancestor (x.) from whence the divergent Elasmo- 
branch, Ganoid, and Amphibian stems have been derived, we may infer that it possessed 
an amphistylic skull, perhaps not differing essentially from that of the recent Noti- 
danus; a persistent notochord supporting dorsally a series of neural and intercalated 
arches, which were related to the anterior and posterior roots of the spinal nerves, as 
in the embryo Selachian and in the adult Marsipobranch, and also in relation ventrally 
with a similarly double series of haemal arches enclosing the dorsal aorta ; that re- 
spiration was carried on by external gills, which were developed in connexion with 
the mandibular as well as from the other visceral arches, and were probably destitute 
of an opercular covering ; persistent segmental openings may have been present, and 
also an archipterygium composed of three proximal segments (propterygium, meso- 
pterygium, and metapterygiiun), each supporting a series of radial cartilages, but 
