MR. T. W. BRIDGE ON THE OSTEOLOGY OF POLYODON FOLIUM. 
723 
none. Skin in many furnished with osseous scutes, or with rhomboidal scales, in some 
naked, save on the prolongation of the body along the upper lobe of the caudal fin.” 
Though Dr. Traquair may have adduced sufficient evidence to justify the separa- 
tion of Paleeoniscidse and Platysomkke from the Lepidosteoid forms with which they 
have so long been associated, nevertheless I venture to think that he has not been 
so successful in his efforts to establish the claims of those two families to be included 
in the same' group with the Acipenseroid forms. Most of the characters on which he 
relies for justifying the union of the Paleeoniscidse with the Selachoid Ganoids, are 
certain generalised structures which are common to widely diverse groups of Fishes, and 
the force which these facts possess as evidences of affinity is very considerably lessened 
by considerations based upon more specialised structures. For example, paired inter- 
clavicles are common not only to the special groups under discussion, but also to the 
recent and extinct Crossopterygidm ; segmentation of the interspinous bones or cartilages 
is characteristic of nearly all Ganoids, of Ceratodus, and of Elasmobranchs ; while the 
fin-rays are more numerous than the bones or cartilages which support them in such 
diverse forms as certain Crossopterygii, Ceratodus, and Lepidosiren, and the same may 
be said of the corneous fibres which represent those structures in the Holoceplmli and 
Elasmobranchii. Similar remarks might be made on the persistence of the notochord, 
the absence of ossified ribs, and the condition of the caudal fin. On the other hand, the 
more specialised structures of the Palseoniscidse point strongly to the conclusion that 
they are more closely related to the Teleosteoid than to the Selachoid Ganoids. The 
dermal armature of Palseoniscidae is essentially the same as that of the Crossopterygii 
and the Amiidse, and the latter groups further agree with the former in possessing a 
complete series of opercular bones. The hyomandibular of Palceoniscus is strikingly 
like that of Polypterus, and the former also resembles the latter in the character of the 
preoperculum which extends forwards over the cheek towards the orbit, as well as in 
the possession of a transverse chain of supra-temporal ossicles behind the posterior 
margin of the cranial buckler. 
And in addition, the Palseoniscidse possess well-ossified, quadrate, metapterygoid, 
and articular elements : and there is also evidence that the chondrocranium was not 
altogether devoid of ossific centres. Most of the characters which appear to support the 
view that the Paleeoniscidse are closely related to Polyodon and Acipenser seem to be 
only evidence of generalisation or the persistence of ancestral features common alike 
to these forms and to many other Ganoids, and not of any special affinity between 
any of the groups which may happen to possess them. But far outweighing the 
evidence in favour of such affinity is that furnished by an examination of the structure 
of the upper jaw in Palceoniscus and its allies. Dr. Traquair has shown that in that 
genus the upper jaws somewhat resemble the same parts in Polyodon, and notably in 
possessing long parosteal pterygoid bones, between which and the backwardly-produced 
maxilla the levator mandibularis muscle must have passed precisely as was described 
in the earlier part of this paper as existing in Polyodon. Nevertheless, in Palseoniscus 
