724 
MR. T. W. BRIDGE ON THE OSTEOLOGY OF POLYODON FOLIUM. 
it would appear that the pterygoids were attached to the prefrontal region, as is the 
case in all the Telesteoid Ganoids, but differed in this respect from the disposition of 
those parts in Polyodon and its allies. While, therefore, it may be admitted that 
the Palseoniscidse and the Platysomidse are to some extent annectent forms between 
the two main subdivisions of the Ganoids, yet at present the balance of evidence is 
against their being included with the Selachoid forms, and in favour of their classi- 
fication with the Teleosteoidei, though whether they should provisionally be classed 
with the Lepidosteoicl forms, or form the nucleus of a new group is not, as yet, quite 
clear. 
A careful comparison of the dermal armature of the Placodermi and the Cephalaspidse 
with that of Polyodon and Acipenser, failed to suggest the existence of any near 
relationship between these widely different forms, or to throw any light on the nature 
of the cranial buckler in the two Pal'sezoic families. 
D. Comparison of Polyodon with the Amphibia. 
A comparison of the skull of Polyodon with the skulls of the Anura and Urodela 
leads to interesting though somewhat perplexing results, which are not without 
significance in relation to the phylogeny of the three most important groups of 
Ichthyopsida. The majority of living Ganoidei, both in their osteology and myology, 
exhibit many indications of affinity to the Amphibia, and Polyodon is no exception to 
this rule. There are several interesting points of agreement with the Urodela. The 
fenestration of the roof of the periotic capsule in Polyodon is comparable in position 
and in its relations to the arch of the posterior vertical semicircular canal to a similar 
vacuity, which in Siren laeertina represents the primitive involution to form the 
internal ear ; and it is noteworthy that in this point both forms agree with each 
other and differ from the Plagiostomi where the parietal foramina are situated to 
the inner or mesial side of the vestibular sac and its canals. Polyodon, in common 
with all other Ganoids, agrees with the majority of Urodela in the absence of a basi- 
hyal and the attachment of the hypohyals to the first mesobranchial segment. With 
the Anura, Polyodon agrees in the ossification of its lower labial cartilages as mento- 
Meckelian bones. But perhaps the most remarkable feature in which Polyodon 
resembles the Anurous Amphibia is the possession of a forwardly directed “ orbitar 
process ” associated with a suspensorium so much inclined backwards that the angle 
of the gape is extended even beyond the posterior limits of the skull. The Marsipo- 
branchii are the only other adult Ichthyopsida in which this singular process has as 
yet been discovered. So far as I am aware no such structure, or even a rudiment of 
it, has yet been detected in any of the Elasmobranchii, the Teleostei, or in any of the 
Urodela, either in the adult or in the embryo. The precise significance of the 
structure in question is not very clear, but one suggestion may be made. The con- 
dition of the “orbitar process” as an apparently functionless rudiment in Polyodon 
