MR, T. W. BRIDGE ON THE OSTEOLOGY OF POLYODON FOLIUM. 725 
does not throw any light on its primary origin or function ; hut its existence and 
relations in the adult Lamprey, as well as its transitory condition in the developing 
Frog, suggest that originally it may have acted as a kind of anterior suspensor to the 
much inclined mandibular pier of such primitive Vertebrata as happened to possess a 
suctorial mouth, prior to the adaptation of the palato-pterygoid bar to that purpose. 
Hence the existence of an “ orbitar process ” in the permanently suctorial mouth of 
the Marsipobranchii, and as a rudiment in the temporarily suctorial mouth of the 
young Tadpole. These facts, coupled with the rotation of the quadrate cartilage, 
which from the direction and relations of the “ orbitar process ” we may infer to have 
taken place, are sufficient to render extremely probable the existence of a close 
parallelism between the developmental history of the cranium in the embryo Polyodon 
and such otherwise dissimilar forms of Anurous Amphibia as Bufo ornatus, Dactyletlira 
capensis, Rana temporaries and Otilophus margaritifer, though I am not aware that 
anything has yet been made known about the cranial embryology of the former. 
The existence in this very generalised Ganoid of so remarkable a combination 
of Selachian and Amphibian characters at once suggests an inquiry into the probable 
ancestral relations to the progenitors of those two groups, and the first question for 
solution is, whether the Ganoids are more closely related phylogenetically to the 
Amphibia or to the Elasmobranchii ? The embryology of the Ganoids is at present 
altogether unknown, and from this source, at all events, no clue can be obtained as to 
the phylogeny of the group ; consequently we are dependent upon such conclusions as 
may be drawn from comparative anatomy. An analysis of the structural features 
common to any two of the three groups seems to me to lend very strong support to 
the view that the Ganoids and the Amphibia are of monophyletic origin. 
The two groups agree with each other in — 
I. The possession of morphological or functional lungs. 
II. The rudimentary condition of the cerebellum. 
III. The persistence of the embryonic connexion between the efferent renal ducts 
(Wolffian ducts) and the Mullerian ducts, due to the incomplete splitting of the 
primitive segmental duct. 
IV. The development of parosteal and endosteal ossifications in relation with their 
originally cartilaginous crania. 
V. The possession of a conus arteriosus with valves. 
VI. The existence of functional external gills in the embryos. 
In all of these points both the Urodela and the Anura agree with the Ganoids, 
while the anatomical facts to which reference is made in paragraphs III., V., and VI., 
are common alike to the Ganoids, the Amphibia, and the Elasmobranchs. 
The Urodela and the Ganoids have also certain minor characters in common, which 
are not shared by the Anura, and these are — the possession of a dentigerous splenial, 
the fenestration of the roof of the otic capsule ( Polyodon and Siren lacertina), the 
absence of a basihyal and the attachment of the hypohyals to the first basibranchial 
