MR, T. W. BRIDGE OR THE OSTEOLOGY OF POLYODOR FOLIUM. 
729 
entirely devoid both of morphological and functional lungs. Such an ancestral type 
may be supposed to have very early differentiated into the two groups of Apneuma- 
toccela and Pneumatocoela, the former becoming the root-stock of the modern Elasmo- 
branchii, which have retained not a few of the characteristics of their prototype, while 
the latter, by acquiring rudimentary and more or less functional lungs, became the 
primitive double-breathers from which have been derived the Ganoids and Amphibia. 
Of these primitive Pneumatocoela it is possible that the recent Lepido siren, Protopterus, 
and Geratodus are the nearest, though much modified, living allies. The Amphibia 
developed more complex lungs and a chiropterygium, and became adapted to a 
terrestrial life, while what remained of the Ganoid-Amphibian stem after the sepa- 
ration of the Amphibia, differentiated into the Teleosteoid Ganoids, the Teleostei 
proper, and into the progenitors of the Selachoid Ganoids, all of which acquired an 
ichthyopterygium, and in the more specialised forms the originally complex and more 
or less respiratory primitive lungs became much simplified in structure and mainly 
mechanical in function. If the view which I have here ventured to suggest be correct, 
then we may regard these forms as being the relatively little modified descendants 
of the common stock from which the other Ganoids and the Teleostei have been 
differentiated, and as bearing a relation to the latter comparable to that in which the 
Tragulina stand to all the other recent ruminating Artiodactyla. That the Selachoid 
Ganoids have departed less from the ancestral type ( x .) than any of the other groups, 
with the exception of the Selachii, is evinced by the structure of their vertebral column, 
fins, and chondrocranium, by their retention of mandibular gills, and by several other 
less important features. Two facts in the cranial structure of Polyodon are not easy to 
explain, viz., the formation of the upper jaw, and the existence of the “ orbitar process.” 
The union of the pterygoid processes in a median symphysis may have been the 
primitive condition of the jaw in the ancestral form ( x .) and in the primitive Pneu- 
matoccela, but that while persistent in Elasmobranchs and in Polyodon , it has been 
superseded by a different arrangement, viz., the union of the pterygoid processes with 
retral palatine outgrowths, in all other Ganoids and in all Teleostei and Amphibia ; or 
we may conceive that, whatever may have been the primitive condition of these 
structures, the development of similarly constituted jaws in the Shark and in Polyodon 
is simply due to a parallelism in adaptive modification, though this latter alternative 
is far less probable. Neither is it easy to account for the retention of the “orbitar 
process ” by Polyodon. It may have been an adaptive modification, correlated with a 
suctorial mouth in the larval forms of those Ganoids that were first differentiated 
from the Ganoid- Amphibian stem, and independently developed ; or, and this is more 
probable, it may have been possessed by certain primitive suctorial-mouthed Pneuma- 
tocoela, but has become obsolete in all their descendants except Polyodon and the 
embryos of the Anurous Amphibia.'' 5 ' 
* The probability that tlie primitive common ancestors of the Ganoids and Amphibians possessed 
suctorial mouths seems to me to be very great, and if this view be correct, the curious differences which 
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