730 
MR, T. W. BRIDGE ON THE OSTEOLOGY OF POLYODON FOLIUM. 
Thus it would seem that the Polyodontidse constitute a remarkably central group 
of Fishes. They resemble the Elasmobranchs in retaining many of the features which 
were characteristic of their presumptive common ancestor; and they further resemble 
the Amphibia and the higher Ganoids in having undergone a certain amount of 
specialisation in the same direction, and that they are not altogether without indi- 
cations of having undergone a certain amount of retrogression is evident from the 
localised distribution of their rhomboidal scales. 
Haeckel’s'" pedigree of the “ non-amnionate craniota” is somewhat different from 
that which I have ventured to suggest. He appears to regard the Amphibia, the 
Dipnoi, and the Halisauria as constituting one line of descent from a primitive 
pre-Selachian stem, while the Ganoids and the Teleostei are derivatives of the 
group of Plagiostome Elasmobranchs, the Chondrosteous Ganoids forming a transition 
type between the latter and the higher Ganoids. If this view be correct, we are 
required to believe that morphological or functional lungs have been developed quite 
independently on precisely the same plan, and with every grade of transition between 
their extreme modifications in two diverging groups of Ichthyopsida, modified for 
entirely different habits of life. The a priori improbability of such a course being 
adopted seems to me to be very great. The complex double swim-bladder of the 
Crossopterygian Ganoids is connected by a gradual transition in almost every 
anatomical detail with the simple, undivided air-bladder of the Physoclisti on the 
one hand, and through the double, cellular and functional lungs of the Dipnoi, with 
the more perfect and exclusively air-breathing structures of the Amphibia on the 
other. The best explanation of such facts seems to me to be the assumption that the two 
extremes of lung structure have been derived by progressive modification in opposite 
directions from some intermediate type of structure which, as I have suggested, may 
have been possessed by the primitive Pneumatocoela. Such evidence in favour of the 
monophyletic origin of the Pneumatocoela drawn from the consideration of a single 
organ, is, as I have tried to show, supported and strengthened by the existence of 
numerous other points of agreement, in both osteology and myology, between their two 
main groups of descendants. Moreover, if we accept Haeckel’s views on the ancestral 
relations of the Dipnoi, it becomes exceedingly difficult to explain the existence of 
numerous points of agreement with the Crossopterygidse which is so striking a feature 
in the anatomy of those Fishes. On the other hand, if the Dipnoi are looked upon 
as the much modified descendants of the primitive Pneumatocoela, there is much less 
difficulty in explaining their affinities to the early Ganoids in the structure of the 
vertebral column and fins, and their Amphibian tendencies, which are so clearly shown 
in the structure of their lungs, heart, nostrils, &c. 
exist in the cranial metamorphoses of the larval Urodela and Anura are to some extent explicable. The 
embryos of Polyodon and the Anura retain for a time this primitive suctorial mouth, but in the larval 
Urodela it has been replaced by a masticatory mouth, the suctorial stage being altogether suppressed, 
*■ “Naturliche Schdpfungsgeschichte,” 1873. 
